IndexRearrangementForStreams needs only a small subset of the Communicator's functionalities.
This change also makes testing easier. There is no need to derive from the concrete class Communicator to test IndexRearrangementForStreams anymore.

Single Responsibility...

An empty vector sendIndicesForCommAfterFtoCPositions is now a valid option

and refactore this function for better readability

and refactoring this function for better readability