ENH: mergedSurf helper class (fixes #104)

- For merging meshedSurf content from parallel sources.
  Ensures zoneIds are properly preserved for sampling in parallel.

Current state
~~~~~~~~~~~~~

Current producers of the region information:
 * sampledTriSurfaceMesh

Current consumers of the region information:
 * nastran writer. The zone ids passed through as PSHELL Ids (with offset 1).

Limitations

  The per-face region association is preserved, but the face/element
  sort order gets lost in reconstruction. Would need to attach
  additional information to the sampled surface and use that for
  sorting, but this would also imply that sampled values be written
  indirectly (or resorted) too to match the order. Zone ids are passed
  through, but not their names. After reconstruction, zone ids are no
  longer contiguous. Re-sorting (as mentioned above) would solve this
  too, but again at the cost of more complexity when writing.