PV3FoamReader - code cleanup

 - streamlined code somewhat, minor attempt to reclaim some memory
 - now use "mesh parts" for patches/zones/sets/etc throughout to avoid
   ambiguity with mesh regions
 - collect superCells and addPointCellLabels in a class. The old version
   actually seemed to have overwritten the addPointCellLabels with each
   cellSet/cellZone. This means that part of the pointFields would be trashed
   in the combination of polyhedral cells, cellSets/cellZones and internalMesh
 - polyDecomp information for muitiple mesh regions, but not yet exploited
 - pointFields now working for cellZones/cellSets
 - extroplating fields onto walls also works as expected for interpolated
   pointFields
 - added tooltips to reader GUI

TODO:
 - pointFields (real and interpolated) for faceSets/faceZones