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

- parallel output.

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

- align the input parameters with those of vtkCloud so that we can
  specify the ASCII precision and the padding width for the output
  file names as well.

- emit TimeValue field, support file series generation

- support internal or boundary meshes, combining the result into a vtm
  file.

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