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/Geometry/Scaling.h

@@ -128,6 +128,20 @@ public:
     return *this;
   }
 
+  /** \returns \c *this with scalar type casted to \a NewScalarType
+    *
+    * Note that if \a NewScalarType is equal to the current scalar type of \c *this
+    * then this function smartly returns a const reference to \c *this.
+    */
+  template<typename NewScalarType>
+  typename ei_cast_return_type<Scaling,Scaling<NewScalarType,Dim> >::type cast() const
+  { return typename ei_cast_return_type<Scaling,Scaling<NewScalarType,Dim> >::type(*this); }
+
+  /** Copy constructor with scalar type conversion */
+  template<typename OtherScalarType>
+  explicit Scaling(const Scaling<OtherScalarType,Dim>& other)
+  { m_coeffs = other.coeffs().template cast<OtherScalarType>(); }
+
 };
 
 /** \addtogroup GeometryModule */