ENH: delay writing of ensight case until after geometry/fields (#2512)

- in situations where the simulation diverges, the ensight writing can
  be incomplete. If the case file is updated prior to writing geometry
  or fields, the generated case may refer to incomplete entries (which
  make loading problematic).

  NOTE: if multiple fields are sampled and written, this change cannot
  entirely prevent case files addressing corrupt fields. For example,

  1a. write U field, update case file with new times/fields
  1b. write p field, update case file with new times/fields
  2a. write U field, update case file with new times
  2b. write p field, but fails

  Since 2a already updates the case file with a new time-step entry
  (for the U field), the case glob patterns will automatically include
  the not-yet-written 'p' field. If this write fails with an
  incomplete/corrupt field, the case file will still be addressing it!