- provide a globalIndex::calcOffsets() taking an indirect list, which
  enables convenient offsets calculation from a variety of inputs.

- new CompactListList unpack variant: copy_unpack()
  The copy_unpack() works somewhat like std::copy() in that it writes
  the generated sublists to iterator positions, which makes this
  type of code possible:

      CompactListList<label> compact = ...;
      DynamicList<face> extracted;

      compact.copy_unpack<face>
      (
          std::back_inserter(extracted),
          labelRange(4, 10)
      );

  -and-

      const label nOldFaces = allFaces.size();
      allFaces.resize(allFaces + nNewFaces);

      auto iter = allFaces.begin(nOldFaces);

      iter = compact.copy_unpack<face>(iter, /* selection 1 */);
      ...
      iter = compact.copy_unpack<face>(iter, /* selection 2 */);

ENH: globalIndex resize()

- can be used to shrink or grow the offsets table.
  Any extension of the offsets table corresponds to 'slots'
  with 0 local size.