ENH: use meshedSurf API for surface writers (issue #104)

- Allows passing of additional information (per-face zone ids) or possibly
  other things, while reducing the number of arguments to pass.

- In sampledTriSurfaceMesh, preserve the region information that was
  read in, passing it onwards via the UnsortedMeshSurface content.

  The Nastran surface writer is currently the only writer making use
  of this per-face zone information.
  Passing it through as a PSHELL attribute, which should retain the
  distinction for parts. (issue #204)