[bugfix] Don't store return value of GridView::begin in a const Entity&

The return value (i.e., the iterator) returned by GridView::begin is a temporary.
Storing only a reference to the object it points to can and does lead to crashes
when building with optimization.

(cherry picked from commit 0feafa45535d4959f22aeaeaec35e0ee1fc9cabc)
Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>