PV3FoamReader repaired several misfeatures

  - On the first call, select 'internalMesh' and the 'p', 'U' fields.
    This corresponds to a fairly common default case without loading all
    possible fields etc.
  - Include zones should now work without issuing a FatalError when the
    corresponding files are missing. I must have tried too many things
    at once and didn't notice this before.
  - Use ZoneMesh for zone names if the fvMesh is already cached
    (skips re-reading the same files).
  - Change reader description to "OpenFOAM Reader" to distinguish it a bit
    from the vtkOpenFOAMReader built into paraview/vtk
  - Misc. file and method names reorganized

  - Remaining problem:
    mixing internalMesh and cellZones is clunky since there aren't yet
    interpolated point values for cellSets/cellZones. Paraview complains
    about the mismatch, but extracting a block is still okay.

  TODO:
  - multi-port output for Lagrangian
  - hold all the vtk mesh parts (including multiblock) and just pass the
    reference. With this, the pipeline might notice that the geometry is
    actually identical and skip the PVGeometryFilter that takes so long.