IsDenseMatrix for transposed matrix-view

Summary

This MR adds operator[i][j] access to TransposedMatrixView that swaps the indices when accessing the wrapped matrix. Additionally it provides size information .N() and .M() again with swapped information from the underlying wrapped matrix. The element access is only implemented if the wrapped matrix is dense. The traits that classifies a dense matrix is better documented such that also the TransposedMatrixView can be seen as a dense matrix if the underlying wrapped matrix is dense.

dune/common/transpose.hh

@@ -81,9 +81,9 @@ namespace Impl {
   // Wrapper representing the transposed of a matrix.
   // Creating the wrapper does not compute anything
   // but only serves for tagging the wrapped matrix
-  // for transposition. This class will store M by value.
-  // To support reference-semantic, it supports using
-  // M=std::reference_wrapper<OriginalMatrixType>.
+  // for transposition. This class will store the matrix
+  // of type `Matrix` by value. To support reference-semantic,
+  // it supports using Matrix=std::reference_wrapper<OriginalMatrixType>.
   template<class Matrix>
   class TransposedMatrixWrapper :
     public TransposedMatrixWrapperMixin<ResolveRef_t<Matrix>>