GitLab

- includes initial relocation of low-level vtu handling.
  Extracted, refactored from the Catalyst function object.

STYLE: adjust style of HashTable iterator pointer dereference

- removed fvMesh sub-channels if favour of selector flags.
  To enable/disable portions of the geometry:

      internal    true/false
      boundary    true/false

  This replaces the previous method of sub-channels in which restrictions
  were propagated from the pipeline through.

  Before
  ======
      Function Object:

          input
          {
              fields    (U p);
          }

      Pipeline:
          input = coprocessor.CreateProducer(datadescription, 'input/patches')

  Now
  ===
      Function Object:

          input
          {
              internal  false;
              fields    (U p);
          }

      Pipeline:
          input = coprocessor.CreateProducer(datadescription, 'input')

- previously had separate function objects for each category of source
  (finite volume, finite area, lagrangian). This lead to isolation of
  the individual input sources, which highly restricted the flexibility.

- now have a single catalyst function object with an arbitrary number
  of inputs (sources). The catalyst channel name is that of the
  corresponds dictionary name. For volume mesh sources, it is possible
  to address "mesh" and "patches" sub-channels.

  Eg,
      mesh = coprocessor.CreateProducer(datadescription, 'myregion/mesh')

  This restructuring also aims at integration of other OpenFOAM content
  (eg, surfMesh, sampled probes, sampled surfaces) as a future
  development.

- the output directory (by default "<case>/insitu") is passed onto
  catalyst as its working directory, using the upstream patches that
  now close issue #4

- reflects current change being made to the vtkCPProcessor API to
  include chdir capabilities.

- Note that stopping the catalyst function object via a `timeEnd` value
  does not actually call the corresponding end() method.

  See: Development/OpenFOAM-plus#791

ENH: support mkdir as dictionary keyword

- this is a convenient means of creating an output directory from within
  the function object without corresponding python or shell script.

  Some pipelines (image generation) balk if the output directory does
  not exist. Others (vtm writer etc) will create their own.

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