Add template alias ConstCopyOrReference

This adds constness to the stored copy or reference.
Notice that simply using CopyOrReference<const T> for
ConstCopyOrReference<T> does not work since 'const T'
is the same as T for reference types because all references
are const.

dune/solvers/common/copyorreference.hh

@@ -17,7 +17,7 @@ namespace Solvers {
 /**
  * \brief A wrapper storing either a copy or reference
  *
- * \tparam T Type to be refered to
+ * \tparam T Type to be stored
  *
  * If T is of reference type, the wrapper stores a reference
  * to the passed object. Otherwise it stores a copy.
@@ -114,6 +114,27 @@ private:
 
 
 
+/**
+ * \brief A wrapper storing either a const copy or const reference
+ *
+ * \tparam T Type to be stored
+ *
+ * This is an alias for CopyOrReference<TT> where TT
+ * is either 'const T' if T is a raw type or 'const S&'
+ * if T is a 'S&'. Notice that in the latter case this
+ * is different from 'const T' which would be the same
+ * as T.
+ */
+template<class T>
+using ConstCopyOrReference =
+  CopyOrReference<
+    std::conditional_t<
+      std::is_reference<T>::value,
+      const std::remove_reference_t<T>&,
+      const T>>;
+
+
+
 } // end namespace Solvers
 } // end namespace Dune