ENH: provide separate geometry description per region/patch (issue #278)

- Also fixed bug noted in issue #269

- Previous implementation had all faces together, which made
  it difficult (impossible) for external applications to
  figure out which geometry was being referred to.

- Provide separate region/patches as follows:

    // Patch: <regionName> <patchName>

  For example,

    // Group: coupleGroup
    // Patch: heater minY
    8( ... )

  The region-name is always present, even if there is only one region.

- This change is a partial reversion to the behaviour in 2.4.x, except
  that we can now also handle multi-region geometries.

  Changing the leading comment from "# " to "// " facilitates parsing
  of the files with OpenFOAM itself if necessary.