[parallel,release] Removed superfluous typedef and documented another one in Interface.
[parallel,release] Adds a new buffered communicator that does not need to know the data amount per index when receiving.

During communication of data with a parallel DUNE grid the amount of data items
received at an index is not known at the receiving side. With this communicator
it is possible to build such a communication based on index lists at the sending
and receiving side. With this patch only fixed size of data items per index is
supported.
[parallel,release] Added implementation for the case of varying number of data per index.

With this patch VariableSizeCommunicator now really supports a varying number of data entries
per index during communication.