- explicit use of UPstream::worldComm in globalIndex methods
  for more clarity

- adjust method declaration ordering:
  de-emphasize the processor-local convenience methods

- consistent use of leading tag dispatch,
  remove unused enum-based dispatch tag

- add begin()/cbegin() with offset (as per List containers)

BUG: missing use of communicator in globalIndex gatherNonLocal

- does not affect any existing code (which all use worldComm anyhow)