Safeguarded some Transform functions with compile time asserts.

Added missing static Identity() to Rotation2D, AngleAxis.

Eigen/src/Geometry/AngleAxis.h

@@ -142,6 +142,8 @@ public:
     m_angle = Scalar(other.angle());
   }
 
+  inline static const AngleAxis Identity() { return AngleAxis(0, Vector3::UnitX()); }
+
   /** \returns \c true if \c *this is approximately equal to \a other, within the precision
     * determined by \a prec.
     *