Mark OLESEN authored Nov 08, 2022 and Andrew Heather committed Nov 08, 2022

- support globalIndex for points/faces as an output parameter,
  which allows reuse in subsequent field merge operations.

- make pointMergeMap an optional parameter. This information is not
  always required. Eg, if only using gatherAndMerge to combine faces
  but without any point fields.

ENH: make globalIndex() noexcept, add globalIndex::clear() method