ENH: PDRsetFields utility (#1216)

- the PDRsetFields utility processes a set of geometrical obstructions
  to determine the equivalent blockage effects.

  These fields are necessary inputs for PDRFoam calculations.

  After setting up the geometries, the -dry-run option can be used to
  generate a VTK file for diagnosis and post-processing purposes.

- this is an initial release, with improvements slated for the future.

NOTE
  - the field results may be less than fully reliable when run in
    single-precision. This howver does not represent a realistic
    restriction since the prepared fields target a combustion
    application which will invariably be double-precision.