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...

A Communicator is now passed to IndexRearrangementForStreams (Commit #7e1ef552). In the constructor of this Communicator MPI_Init is called. Therefore, an additional call to MPI_Init is not needed anymore.

A Communicator is now passed to IndexRearrangementForStreams (Commit #7e1ef552). In the constructor of this Communicator MPI_Init is called. Therefore, an additional call to MPI_Init is not needed anymore.