Add concepts.

dune/matrix-vector/concepts.hh

+#ifndef DUNE_MATRIX_VECTOR_CONCEPTS_HH
+#define DUNE_MATRIX_VECTOR_CONCEPTS_HH
+
+// though not needed for this file, checking
+// for a concept requires this include anyway
+#include <dune/common/concept.hh>
+
+namespace Dune {
+namespace MatrixVector {
+namespace Concept {
+
+struct HasBegin {
+  template <class C>
+  auto require(C&& c) -> decltype(c.begin());
+};
+
+
+struct HasN {
+  template <class C>
+  auto require(C&& c) -> decltype(c.N());
+};
+
+struct HasResize {
+  template <class C>
+  auto require(C&& c) -> decltype(c.resize(0));
+};
+
+struct HasSize {
+  template <class C>
+  auto require(C&& c) -> decltype(c.size());
+};
+
+} // end namespace Concept
+} // end namespace MatrixVector
+} // end namespace Dune
+
+#endif // DUNE_MATRIX_VECTOR_CONCEPTS_HH