ENH: overhaul ensight handling (#1579)

- includes restructuring and simplification of low-level ensight part
  handling and refactor of backends to improve code reuse.

foamToEnsight
-------------

  * new cellZone support.
    This was previously only possible via a separate foamToEnsightParts
    utility that was not parallelized.

  * support for point fields.

  * `-nearCellValue` option (as per foamToVTK)

  * data indexing now uses values from the time index.
    This is consistent with the ensightWrite function object and
    can help with restarts.

  * existing ensight directories are removed, unless the -no-overwrite
    option is supplied

foamToEnsightParts
------------------
  * now redundant and removed.

ensightOutputSurface (new class)
--------------------------------
  * a lightweight wrapper for point/face references that is tailored
    for the ensightSurfaceWriter. It uses compact face/point information
    and is serial only, since this is the format requirements from the
    surfaceWriter class.

ensightMesh (revised class)
---------------------------
  * now only holds a polyMesh reference, which removes its dependency
    on finiteVolume and allows it to be relocated under fileFormats
    instead of conversion.

Removed classes: ensightParts, ensighPartFaces, ensightPartCells

- these were used by foamToEnsightParts, but not needed anymore.