[wip] Make operator* for FieldMatrix more generic

This implements A*B if one of both matrices is a FieldMatrix. The result-type is always a FieldMatrix.

• If A is a FieldMatrix, then C=A*B is implemented by applying B.mtv(A[i],C[i]) for all rows of A and C.
• If B is a FieldMatrix, then C=A*B is implemented by applying A.mv(A_j,C_j) for all columns of A and C. Column access is implemented using a new wrapper class in Dune::Impl::.