ENH: be more forgiving when reading ensight surface files (#1511)

- The case files may contain #... comment lines
- The geometry file may contain an optional "extents" entry
- Properly handle element id specifications (off|assign|ignore|given).

- Partially handle node id specifications (off|assign|ignore|given).
  Treat "given" like "ignore", since results in the lightest amount of
  coding and in many cases the "given" node ids are in fact 1-based
  contiguous values and thus no different than "ignore" for our
  purposes.