YawSteering generalized pointing celestial body

I would propose to generalize the sun variable in YawSteering constructor to be called differently as it is not mandatory to set the Sun. I would propose to add a check that the pointing axis does not collide and we don't have "Impossible to normalize Zero vector" error.

As Forum discussion https://forum.orekit.org/t/offset-on-groundpointing-and-yawsteering/4123/4