- dropped auto-vivification for now (performance issue), but reworked to
  allow easy reinstatement
- derived both iterator and const_iterator from iteratorBase and use
  iteratorBase as our proxy for non-const access to the list elements.
  This allows properly chaining assignments:
     list[1] = list[2];
     list[1] = list[2] = 10;
- assigning iterators from iteratorBase or other iterators works:
     iterator iter = list[20];
- made template parameter nBits=1 the default

    - convertPointField caused a seg-fault if the internal field was null
      - attempted to name the vtk object the name of the null field

+ minor clean-up

- the bit counting is relatively fast:
  under 0.2 seconds for 1M bits counted 1000 times

- trim()'ing the final zero elements tested for a few cases,
  but might need more attention

    - incompressible, Boussinesq variant of buoyantSimpleFoam
    - requires new fixedFluxBoussinesqBuoyantPressure bc on pd at walls
      to balance the flux generated by the temperature gradient

- ditched PrimitivePatchExtra in favour of a PatchTools class that is
  currently just a collection of static functions. They could equally well
  live within PrimitivePatch itself, but isolated also has its advantages.

- MeshedSurface, UnsortedMeshedSurface now have 'regions' instead of
  'patches' since they are more like a faceZone for meshed surfaces than
  patches. This might avoid confusion at a later stage.

- can use the same syntax for boolList, PackedBoolList and labelHashSet
    if (myHashedSet[x]) ...
    if (myBoolList[x]) ...
    if (myPackedList[x]) ...

- exists() = forward to OSspecific exists(...)
- isDir()  = forward to OSspecific dir(...)
- isFile() = forward to OSspecific file(...)
- IOobjectComponents() - split into instance, local, name following rules
  set out for IOobject.

- added IOobject(path, registry, ...) constructor that uses
  fileName::IOobjectComponents(). This hides the complexity we otherwise need.

- set() handles overflow as max_value() - the principle of least surprise.
- use pointers in iterator to avoid any assignment issues. Allows default
  bitwise copy/assignment.

- added Mattijs' speed tests
- optimized resize() and assignment operators to avoid set() method
- add const_iterator and re-did the proxy handling.

Reading/writing by looping across iterators is still somewhat slow, but
might be acceptable.

largest and smallest representable values.  Accessible directly and via pTraits.

- eliminated previous PackedBitRef class, the iterator does all of that and
  can also be used to (forward) traverse the list
- no const_iterator yet

- Note that PackedList is also a bit like DynamicList in terms of storage
  management and the append() method. Since the underlying storage in
  integer, any auto-vivified elements will also flood-fill the gaps with
  zero.