ENH: revise ensight handling (#1579)

- simplify internals of low-level ensight part handling and refactor
  the backend output routines for more reuse. There is still a fair
  bit of residual ugliness in the details of handling ensight output.

- added cellZone support for ensightMesh

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

* foamToEnsight:

  - added cellZone support, which was previously only available using
    foamToEnsightParts. Will seek to deprecate and remove
    foamToEnsightParts in the future.

  - now also converts point fields