General:
    * -roots, -hostRoots, -fileHandler

Specific:
    * -to <coordinateSystem> -from <coordinateSystem>

- Display -help-compat when compatibility or ignored options are available

STYLE: capitalization of options text

- avoids compiler ambiguity when virtual methods such as
  IOdictionary::read() exist.

- the method was introduced in 1806, and was thus not yet widely used

- behaves the same as the valid() method, but can be queried directly
  like a normal raw pointer and as per std::unique_ptr.
  Eg,

      autoPtr<T> ptr = ...

      if (ptr) ...

- test(), get(), set(), unset() with behaviour as per bitSet,
  to allow easier swapping out of boolList <-> bitSet.

- advanced options are not displayed with -help, but only with
  -help-full, which helps retain a better overview of the standard
  options.  Replaces previous ad hoc suppression of -listSwitches,
  -listRegisteredSwitches etc.

- supports copy append and move append

- already had remove() by iterator, but not by key.
  As per erase() which already worked by iterator or by key.

- as per 018124e3

Changed into warning since e.g. tutorials/combustion/fireFoam/LES/compartmentFire
uses columns of 1D stacks so they've got empty but are still 3D.

- simplifies usage.
  Support syncPar check on names() to detect inconsistencies.

- simplify readFields, ReadFields and other routines by using these
  new methods.

- what was previously termed 'setLargeCellSubset()' is now simply
  'setCellSubset()' and supports memory efficient interfaces.

  The new parameter ordering avoids ambiguities caused by default
  parameters.

  Old parameter order:

      setLargeCellSubset
      (
          const labelList& region,
          const label currentRegion,
          const label patchID = -1,
          const bool syncCouples = true
      );

  New parameter order:

      setCellSubset
      (
          const label regioni,
          const labelUList& regions,
          const label patchID = -1,
          const bool syncCouples = true
      );

   And without ambiguity:

      setCellSubset
      (
          const labelUList& selectedCells,
          const label patchID = -1,
          const bool syncCouples = true
      );

- support bitSet directly for specifying the selectedCells for
  memory efficiency and ease of use.

- Additional constructors to perform setCellSubset() immediately,
  which simplifies coding.

  For example,

      meshParts.set
      (
          zonei,
          new fvMeshSubset(mesh, selectedCells)
      );

  Or even

      return autoPtr<fvMeshSubset>::New(mesh, selectedCells);

- BitSetOps::create(len, locations, on) that generates a bitSet with
  the specified length. The optional 'on' bool can be used to flip the
  logic.

- enclosed with a #ifdef Foam_argList_1712 .. #endif pair
  (defined by default).
  In a later release, this can be disabled by default.

ENH: add input length check for -hostRoots option

- unused, does not run in parallel

- avoid enclosing namespace in fvMeshSubset

- gets the pointer within the list.

- improve alignment of various phasePairKey implementations

- get<label>, get<scalar> instead of readLabel, readScalar, etc.