BUG: PDRsetFields fails with non-orthogonal outer region (fixes #1907)

- first sort the cells into their ijk bins, and restrict testing for
  face orientation to those faces with an owner or neighbour that has
  an ijk bin.

ENH: ensure polyMesh from PDRblockMesh is marked as AUTO_WRITE

- the particular polyMesh constructor inherits the writeOpt, which
  makes is dependent on the caller and thus somewhat fragile for the
  top level caller.