Let Entity return geometry as a copy.

Geometry is lightweight enough and this is what DUNE's grid interface
mandates.

opm/grid/cpgrid/Entity.hpp

@@ -149,7 +149,7 @@ namespace Dune
             }
 
             /// Returns the geometry of the entity (does not depend on its orientation).
-            const Geometry& geometry() const;
+            Geometry geometry() const;
 
             /// We do not support refinement, so level() is always 0.
             int level() const
@@ -431,7 +431,7 @@ unsigned int Entity<codim>::subEntities ( const unsigned int cc ) const
 }
 
 template <int codim>
-const typename Entity<codim>::Geometry& Entity<codim>::geometry() const
+typename Entity<codim>::Geometry Entity<codim>::geometry() const
 {
     return pgrid_->geomVector<codim>()[*this];
 }