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.