Add concept indirectly_copyable to constrain the DenseMatrixAssigner

Summary

Add concept definition Std::indirectly_copyable that checks whether the value provided by one iterator can be assigned to the reference provided by a second iterator. This concept is then used to constrain the DenseMatrixAssigner properly. The old constraint was not precise enough and lead to some unintended constructor invocations.

dune/common/std/test/CMakeLists.txt

@@ -10,6 +10,9 @@ dune_add_test(SOURCES accessorstest.cc
 dune_add_test(SOURCES extentstest.cc
               LABELS quick)
 
+dune_add_test(SOURCES iteratortest.cc
+              LABELS quick)
+
 dune_add_test(SOURCES mappingtest.cc
               LABELS quick)