Static size checks for FieldMatrix::rightmultiply

Previously, the following piece of code would compile and only
only predictable fail at runtime with assertions enabled.

  Dune::FieldMatrix<double, 2, 3> A;
  // populate A

  Dune::FieldMatrix<double, 3, 2> B_bad;
  // populate B_bad
  A.rightmultiply(B_bad);

It now fails to compile.