ENH: improvements for ensightWrite function object (issue #926)

- align input parameters and some of the behaviour with vtkWrite

  The output is now postProcessing/<name> for similar reasoning as
  mentioned in #866 - better alignment with other function objects, no
  data collision with foamToEnsight output.

- separate controls for internal and boundary meshes

- can restrict conversion based on zone names, enclosing volumes,
  bounding box.