ENH: globalIndex gather for PatchTools::gatherAndMerge (#2402)

- lower memory overhead, simpler code and eliminates need for
  ListListOps::combineOffset()

- optional handling of local faces/points for re-using in different
  contexts

STYLE: labelUList instead of labelList for globalMesh mergePoints

STYLE: adjust verbose information from mergePoints

- also report the current new-point location