WIP: Resolve "BufferSystem receiver info without sender ranks"

Closes #121 (closed)

This MR extends the BufferSystem API with a new setReceiverInfo overload
void setReceiverInfo( const uint_t numRanksToRecvFrom, bool changingSize );
that can be used set the number of expected receives without specifying the actual ranks. The changingSize argument is available to provide consistency with the other API functions. However, this MR only implements changingSize=true.

For the realization of this MR a new communication is derived from AbstractCommunication. The first call to waitForNextReceive waits for all size messages and sets up the necessary receives for all payloads. Consecutive calls to this function work similar to KnownSizeCommunication.