PV3FoamReader fixes

  - added printMemory() method to aid debugging memory used
  - call UpdateInformation() if the mesh has changed
  - automatic activation of "internalMesh", "p", "U"
    is suppressed if a fvMesh is cached. Otherwise,
    appearing/disappearing volFields can cause a crash

  Note:
  The message "had to make up a selection value" that sometimes occurs
  seems to be paraview related (see paraview mailing list),