Add covariance consider cd

[danlueck]

Consider Cd for Covariance propagation

• I started to implement the Consider Covariance approach from Freddis thesis
• This was not really part of any ticket or project, I mostly did it on free time
• It is not entirely finished (definitely not tested)
• Taylor expansion and RK4 with new method exist
• RK8 still missing
• Testing definitely missing
• I'm creating this MR now so it doesn't get lost

[danlueck]

I added RK8 integration now so from a neptune prespective this is more or less complete. It definitely requires a thorough review (maybe get Freddy involved, too) and some in depth testing and validation