ENH: refactor paraview readers code to avoid duplication

- as originally intended years ago, but never actually done.

- use 'foamPvCore' instead of 'vtkPVReaders' to avoid potential name
  collisions with any 'vtk*' files and since we may reuse these
  functions in other foam-paraview modules (not just readers).

STYLE: use same font size/colour for patch-names as for point-numbers

BUG: repair issue with single time-step

- paraview time-selector returns '0' as the requested time if there is
  only one time step. However, if we have skipped the 0/ directory,
  this single time step is likely a non-zero value.