ENH: preserve globalIndex merge information within mergedSurf

- for later reuse with fields (for example)

ENH: use 'scheduled' for surfaceWriter field merging (#2402)

- in tests with merging fields (surfaceWriter), 'scheduled' was
  generally faster than 'nonBlocking' for scalars, minorly faster for
  vectors.
  Thus make 'scheduled' the default for the surfaceWriter but with a
  user-option to adjust as required. Previously simply relied on
  whichever default globalIndex had (currently nonBlocking).

  Reuse globalIndex information from mergedSurf instead of
  globalIndex::gatherOp to avoid an extra MPI call to gather sizes
  each time.

  These changes will not be noticable unless surface sampling is done
  very frequently (eg, every iteration) and with large core counts.