IdentityMatrix: Add methods N() and M()

Since it's very difficult to make progress on !52 (merged), I'll start to break it up into smaller, less controversial changes. This is the second piece that I'd also like to get out of the way. The feature is essentially uncontroversial: matrices can be asked for their dimensions through methods N() and M(). Instances of the IdentityMatrix class cannot for some reason. Now they can.

The only good reason against merging this is that I've repeatedly heard the IdentityMatrix should be abandoned altogether. If there's a consensus for that, I'll happily come up with a merge request to deprecate this class instead.

dune/common/identitymatrix.hh

@@ -29,9 +29,9 @@ namespace Dune
    *  Implementation of an identity matrix that does not store any data.
    *
    *  \tparam  K  field type
-   *  \tparam  N  dimension
+   *  \tparam  n  dimension
    */
-  template< class K, int N >
+  template< class K, int n >
   struct IdentityMatrix
   {
     /** \brief field type */
@@ -40,9 +40,14 @@ namespace Dune
     typedef std::size_t size_type;
 
     /** \brief return number of rows */
-    constexpr size_type rows () const { return N; }
+    constexpr size_type rows () const { return n; }
+    /** \brief return number of rows */
+    constexpr size_type N () const { return n; }
+
+    /** \brief return number of columns */
+    constexpr size_type cols () const { return n; }
     /** \brief return number of columns */
-    constexpr size_type cols () const { return N; }
+    constexpr size_type M () const { return n; }
 
     /** \copydoc Dune::DenseMatrix::mv */
     template< class X, class Y >
@@ -133,7 +138,7 @@ namespace Dune
     /** \copydoc Dune::DenseMatrix::frobenius_norm2 */
     typename FieldTraits< field_type >::real_type frobenius_norm2 () const
     {
-      return FieldTraits< field_type >::real_type( N );
+      return FieldTraits< field_type >::real_type( n );
     }
 
     /** \copydoc Dune::DenseMatrix::infinity_norm */
@@ -149,13 +154,13 @@ namespace Dune
     }
   };
 
-  template <class DenseMatrix, class field, int N>
-  struct DenseMatrixAssigner<DenseMatrix, IdentityMatrix<field, N>> {
-    static void apply(DenseMatrix &denseMatrix, IdentityMatrix<field, N> const &rhs) {
-      DUNE_ASSERT_BOUNDS(denseMatrix.M() == N);
-      DUNE_ASSERT_BOUNDS(denseMatrix.N() == N);
+  template <class DenseMatrix, class field, int n>
+  struct DenseMatrixAssigner<DenseMatrix, IdentityMatrix<field, n>> {
+    static void apply(DenseMatrix &denseMatrix, IdentityMatrix<field, n> const &rhs) {
+      DUNE_ASSERT_BOUNDS(denseMatrix.M() == n);
+      DUNE_ASSERT_BOUNDS(denseMatrix.N() == n);
       denseMatrix = field(0);
-      for (int i = 0; i < N; ++i)
+      for (int i = 0; i < n; ++i)
         denseMatrix[i][i] = field(1);
     }
   };