Feature/organize traits

I'd like to keep things organized before they get out of hand.

This puts the traits structs to a dedicated namespace Traits, but leaves the traits utilities in the namespace Dune::MatrixVector. It makes use of the traits utilities in several places (axpy, axy, transformmatrix), which IMHO improves readability a lot. Adds deprecation warnings for the moved files.

Additionally this extend traits that the core modules are lacking, e.g. FieldTraits for ScaledIdentityMatrix.

dune/matrix-vector/matrixtraits.hh

+#ifndef DUNE_MATRIX_VECTOR_MATRIXTRAITS_HH
+#define DUNE_MATRIX_VECTOR_MATRIXTRAITS_HH
+
+#include <dune/matrix-vector/traits/matrixtraits.hh>
+
+#warning \
+  This file is deprecated and might vanish soon. \
+  Please use Dune::MatrixVector::Traits::MatrixTraits \
+  from <dune/matrix-vector/traits/matrixtraits.hh> instead \
+  or the trait utilities in namespace Dune::MatrixVector \
+  to be included via <dune/matrix-vector/traits/utilities.hh>.
+
+namespace Dune { namespace MatrixVector {
+
+template <class T>
+using MatrixTraits = Traits::MatrixTraits<T>;
+
+}}
+
+#endif // DUNE_MATRIX_VECTOR_MATRIXTRAITS_HH