Return a copy for Geomerty::center()

This is mandated by the DUNE interface. It also makes a lot of sense
because now people do not need to think whether
const auto& center = entity.geometry().center();
might cause a dangling reference. It just wont as it is always a copy
and never a reference to a member.

opm/grid/cpgrid/Geometry.hpp

@@ -253,7 +253,7 @@ namespace Dune
             }
 
             /// Returns the centroid of the geometry.
-            const GlobalCoordinate& center() const
+            GlobalCoordinate center() const
             {
                 return pos_;
             }