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/traits/utilities.hh

@@ -17,22 +17,22 @@ namespace MatrixVector {
 // convenience compile-time functions to classify types
 template <class T>
 constexpr auto isNumber() {
-  return Std::bool_constant<IsNumber<T>::value>();
+  return Std::bool_constant<IsNumber<std::decay_t<T>>::value>();
 }
 
 template <class T>
 constexpr auto isScalar() {
-  return Std::bool_constant<Traits::ScalarTraits<T>::isScalar>();
+  return Std::bool_constant<Traits::ScalarTraits<std::decay_t<T>>::isScalar>();
 }
 
 template <class T>
 constexpr auto isVector() {
-  return Std::bool_constant<Traits::VectorTraits<T>::isVector>();
+  return Std::bool_constant<Traits::VectorTraits<std::decay_t<T>>::isVector>();
 }
 
 template <class T>
 constexpr auto isMatrix() {
-  return Std::bool_constant<Traits::MatrixTraits<T>::isMatrix>();
+  return Std::bool_constant<Traits::MatrixTraits<std::decay_t<T>>::isMatrix>();
 }
 
 template <class T>