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.

dune/common/fmatrix.hh

@@ -137,8 +137,11 @@ namespace Dune
     using Base::rightmultiply;
 
     //! Multiplies M from the right to this matrix
-    FieldMatrix& rightmultiply (const FieldMatrix<K,cols,cols>& M)
+    template <int r, int c>
+    FieldMatrix& rightmultiply (const FieldMatrix<K,r,c>& M)
     {
+      static_assert(r == c, "Cannot rightmultiply with non-square matrix");
+      static_assert(r == cols, "Size mismatch");
       FieldMatrix<K,rows,cols> C(*this);
 
       for (size_type i=0; i<rows; i++)