PV3FoamReader gets (single) mesh region handling

  - handling multiple regions require multiple readers
  - a region is currently recognized by the file name, anything after the
    '=' delimiter (eg, "case=region.OpenFOAM") is used to determine the
    mesh region, but might be changed in the future
    eg, 'case%region', 'case^region', 'case~region', 'case{region}' ...

Note:
  - Having a separate reader for each region instead attempting to handle
    all the mesh regions in a single reader is the better solution.
    It is not only simpler, but allows distinct field selections for
    each region
Todo:
  - Haven't a test for Lagrangian and multi-regions.