[fill_array] Don't use std::move() when returning a locally created array.

That inhibits RVO and actually makes the code slower.  And even if the
compiler decides against RVO, the standard requires that he moves from the
object, if possible.  See Scott Meyers "Effective Modern C++", second half of
Item 25.

Thanks Markus Blatt for noticing.

dune/common/array.hh

@@ -58,7 +58,7 @@ namespace Dune
   {
     std::array<T,n> r;
     r.fill(t);
-    return std::move(r);
+    return r;
   }
 
   /** @} */