BackwardCommunication should be an optional part of the interface.

I propose to introduce to add a direction template parameter to the capability canCommunicate because it seems that all major projects (dumux,dune-fem, dune-functions, dune-pdelab), that use the DUNE core do not need or use BackwardCommunication. Inside the DUNE core it's only used in the grid implementation and the corresponding test.

Therefore, new grid implementations should not necessariy have to implement this feature. ForwardCommunication is enough.

This means that the capability canCommunicate should reflect that a grid can do ForwardCommunication. For BackwardCommunication there should be an extra capability or the canCommunicate should get a direction template parameter.