ENH: change from triSurface to meshedSurface storage for iso-surfaces

- this makes it easier to reuse the code, and sampledSurface expect
  a face (not a labelledFace), so this also eliminates a translation
  level and simplifies memory management.

- before/after comparison of the sampled iso-surfaces tested with
  iso-surfaces from interFoam/RAS/angledDuct tutorial (serial and
  parallel)