Feature/smaller cleanups

as the title suggests, these are a couple of smaller fixes, which are not directly related and aren't important enough to manage them in separate branches.

dune/istl/bcrsmatrix.hh

@@ -1623,8 +1623,8 @@ namespace Dune {
}
//! y += alpha A x
-    template<class X, class Y>
-    void usmv (const field_type& alpha, const X& x, Y& y) const
+    template<typename F, class X, class Y>
+    void usmv (F&& alpha, const X& x, Y& y) const
{
#ifdef DUNE_ISTL_WITH_CHECKING
if (ready != built)