ENH: unified catalyst function object (closes #3)

- 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