This reverts commit cc7bb07a.

The refactored version using std::find instead of std::unique does not work for vectors containing a zero as a receiving index.  For example this is the case in the app BoundaryLayer --> revert to the old version.

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