Add smart cast functions and ctor with scalar conversion (explicit)

to all classes of the Geometry module. By smart I mean that if current
type == new type, then it returns a const reference to *this => zero overhead

Eigen/src/Core/util/XprHelper.h

@@ -169,4 +169,9 @@ template<typename ExpressionType, int RowsOrSize=Dynamic, int Cols=Dynamic> stru
   typedef Block<ExpressionType, RowsOrSize, Cols> Type;
 };
 
+template<typename CurrentType, typename NewType> struct ei_cast_return_type
+{
+  typedef typename ei_meta_if<ei_is_same_type<CurrentType,NewType>::ret,const CurrentType&,NewType>::ret type;
+};
+
 #endif // EIGEN_XPRHELPER_H