openfoam: remove hard-coded limits on polyhedral size

- for some systems, can have a so-called "single-cell" OpenFOAM mesh
  with a single polyhedral cell that has LOTS of faces.

  This can be used for mapping surface noise data to retain most of
  the geometry but entirely discarding the internal field. It can also
  potentially arise from finiteArea situations.

- code cleanup on cell matching to improve readability.