- constructor for empty cell/face/point Zones, with contents to be
  transferred in later.

- ZoneMesh::operator(const word&) to return existing zone or a new empty one.

- this provides a better typesafe means of locating predefined cell
  models than relying on strings. The lookup is now ptr() or ref()
  directly. The lookup functions behave like on-demand singletons when
  loading "etc/cellModels".

  Functionality is now located entirely in cellModel but a forwarding
  version of cellModeller is provided for API (but not ABI) compatibility
  with older existing user code.

STYLE: use constexpr for cellMatcher constants

- makes for clearer code

ENH: make writeIfDifferent part of Ostream

- reduces ambiguity between matching a list of strings and a single string.

- when dictionary keywords change between versions, the programmer
  can use these compatibility methods to help with migration.

    * csearchCompat, foundCompat, lookupEntryPtrCompat, lookupEntryCompat,
      lookupCompat, lookupOrDefaultCompat, readIfPresentCompat, ...

  They behave like their similarly named base versions, but accept an
  additional list of older keyword names augmented by a version number.
  For example,

      dict.readIfPresentCompat
      (
          "key", {{"olderName", 1612}, {"veryOld", 240}},
          myscalar
      );

   where 1612=OpenFOAM-v1612, 240=OpenFOAM-v2.4.x, etc.

- If the entry could be directly inserted: a pointer to the inserted entry.
- If a dictionary merge was required: a pointer to the dictionary that
  received the entry.
- Return nullptr on any type of insertion failure.

This change is code compatible with existing code since it only alters
a bool return value to be a pointer return value.

- simplify string output code

- improved memory alignment reduces overhead for Int32 compilation

- added move/swap semantics

- made the type() readonly in favour of setVariant() to allow change
  of variant within a particular storage representation.
  Eg, STRING -> VERBATIMSTRING.

- move append() single element to List and DynamicList

ENH: add stringOps::count to avoid unnecessary string conversions

- support send/receive with embedded '\0' characters

- avoid meshModifier contents from being read immediately upon
  construction, since this recreates an existing modifier instead of
  allowing us to specify our own.

- make single-parameter construct (label) explicit
- consolidate iterators
- slightly reduced overhead for some HashSet types
- improved resizing behaviour
- compact output for empty Ptr hashes

- unused, unmaintained and slower than the regular HashTable

- the zero::null and one::null sub-classes add an additional null
  output adapter.

  The function of the nil class (special-purpose class only used for
  HashSet) is now taken by zero::null.

- consistent with C++ STL conventions, the reverse iterators should
  use operator++ to transit the list from rbegin() to rend().

  The previous implementation used raw pointers, which meant that they
  had the opposite behaviour: operator-- to transit from rbegin() to
  rend().

  The updated version only has operator++ defined, thus the compiler
  should catch any possible instances where people were using the old
  (incorrect) versions.

- updated forAllReverseIters() and forAllConstReverseIters() macros to
  be consistent with new implementation and with C++ STL conventions.

- forAllReverseIters and forAllReverseConstIters macros
- stdFoam::rbegin(), stdFoam::rend()
  stdFoam::crbegin(), stdFoam::crend()

- Instead of relying on #inputMode to effect a global change it is now
  possible (and recommended) to a temporary change in the inputMode
  for the following entry.

     #default   : provide default value if entry is not already defined
     #overwrite : silently remove a previously existing entry
     #warn      : warn about duplicate entries
     #error     : error if any duplicate entries occur
     #merge     : merge sub-dictionaries when possible (the default mode)

  This is generally less cumbersome than the switching the global
  inputMode. For example to provide a set of fallback values.

      #includeIfPresent "user-files"
      ...
      #default value uniform 10;

  vs.

      #includeIfPresent "user-files"
      #inputMode protect
      ...
      value uniform 10;
      #inputMode merge    // _Assuming_ we actually had this before

  These directives can also be used to suppress the normal dictionary
  merge semantics:

     #overwrite dict { entry val; ... }

- patterns only supported for the final element.

  To create an element as a pattern instead of a word, an embedded
  string quote (single or double) is used for that element.
  Any of the following examples:

      "/top/sub/dict/'(p|U).*"     100;
      "/top/sub/dict/'(p|U).*'"    100;
      "/top/sub/dict/\"(p|U).*"    100;
      "/top/sub/dict/\"(p|U).*\""  100;

  are equivalent to the longer form:

      top
      {
          sub
          {
              dict
              {
                  "(p|U).*"     100;
              }
          }
      }

  It is not currently possible to auto-vivify intermediate
  dictionaries with patterns.

    NOK   "/nonexistent.*/value"   100;
    OK    "/existing.*/value"      100;

- full scoping also works for the #remove directive

        #remove "/dict1/subdict2/entry1"

- convenience -diffEtc option that behaves like -diff, but invokes
  foamEtcFile searching

STYLE: modernize code.

- this increases the flexibility of the interface

- Add stringOps 'natural' string sorting comparison.
  Digits are sorted in their natural order, which means that
      (file10.txt file05.txt file2.txt)
  are sorted as
      (file2.txt file05.txt file10.txt)

STYLE: consistent naming of template parameters for comparators

  - Compare for normal binary predicates
  - ListComparePredicate for list compare binary predicates

- similar to word::validate to allow stripping of invalid characters
  without triggering a FatalError.

- use this validated fileName in Foam::readDir to avoid problems when
  a directory contains files with invalid characters in their names

- adjust rmDir to handle filenames with invalid characters

- fileName::equals() static method to compare strings while ignoring
  any differences that are solely due to duplicate slashes

- more consistent naming:
  * Versions that hold and manage their own memory:
      IListStream, OListStream

  * Versions that reference a fixed size external memory:
      UIListStream, UOListStream

- use List storage instead of DynamicList within OListStream.
  Avoids duplicate bookkeeping, more direct handling of resizing.

- The problem occurs when using atof to parse values such as "1e-39"
  since this is out of range for a float and _can_ set errno to
  ERANGE.

  Similar to parsing of integers, now parse with the longest floating
  point representation "long double" via strtold (guaranteed to be
  part of C++11) and verify against the respective VGREAT values for
  overflow. Treat anything smaller than VSMALL to be zero.