constructor is explicit now

[[Imported from SVN: r1435]]

grid/common/geometry.hh

@@ -110,7 +110,7 @@ namespace Dune
     }
 
     //! copy constructor from GeometryImp
-    Geometry(const GeometryImp<mydim,cdim,GridImp> & e) : realGeometry(e) {};
+    explicit Geometry(const GeometryImp<mydim,cdim,GridImp> & e) : realGeometry(e) {};
   };
 
   //! Specialization of Geometry for mydim=0 (vertices)