ENH: overhaul of channel handling. Various bug fixes. (issue #6)

- 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')