openfoam: improve zone addressing and handling

- add a separate cache handling for id lists instead of storing it
  directly on the VTK mesh. Improves flexibility, but also it is
  quite misleading to displace a "CellID" entry with these are local
  ids that have been assembled from different processors.

- with this change, can now update fields on cellZones and faceZones
  even after turning off internalMesh later.
  The implementation currently ignores cell-to-point interpolation: to
  be addressed in the near future.

- suppress neighbour-side processor faces in faceZones
  to avoid duplicate representation in the final assembly

Still to do for zones:

  - proper mapping of boundary values (face zones).
    As the first step, use the cell value instead of the patch value.
    Using the patch value is too intertwined with the boundary mesh
    at the moment.

  - cell-to-point interpolation (cell/face zones)

  - GUI selection mechanism.
    A flat selector can cause the list length to explode,
    also need better handling of state changes to avoid unnecessary
    re-reading.

    Another blocking point is presented by the current OpenFOAM file
    format. Since there is no information directly in the header,
    currently need to read (parse!) all of the zone files just to obtain
    the zone names for the reader information vector. This is
    potentially too expensive to be desirable.