polymorphic deletion through a base pointer should not be allowed -> protected destructor (http://www.gotw.ca/publications/mill18.htm)

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

If the numberOfSendIndices is zero no entry should be added to the vector of sendProcessNeighbors. This commit fixes an error in MPI communication for some apps with grid refinement.