GitLab

- use cellCentres() instead of volField equivalent for vtk conversion

- make looping variables more consistent

- use updated Foam::vtk::Tools

- use fieldTypes static information

- can remove directly from the temporary objectRegistry prior to
  converting by field type. Cannot, however, do the same for volFields
  or areaFields since these reside on their respective mesh objects.

- was check(), now query()

- emit/suppress information output based on 'log' setting
  (as per other functionObjects)

This integration represents a collective work
  - CINECA (Simone.Bna@cineca.it)
  - OpenCFD
  - with additional input from KitWare (Andy.Bauer@kitware.com)

NB: requires ParaView or ParaView Catalyst with MPI and Python