fix geodesics transformation

[Kwasniok]

The Christoffel symbols do not transform like tensors. This leads to a correction term when dealing with a covariant derivative.

- {\Gamma^\prime}^i_{ab} {u^\prime}^a {u^\prime}^b = \frac{\partial {x^\prime}^i}{\partial x^ j} \left ( -\Gamma^j_{cd} u^c u^d  - \frac{\partial^2 x^j}{\partial x^c \partial {x^\prime}^e} \frac{{x^\prime}^e}{\partial x^d}  u^c u^d\right )