An API to get geopotential coefficients for analytic perturbation theories

[eggrobin]

One day @lamont-granquist will implement primer vector guidance based to the Principia flight plan, integrated by Principia (see #1659 (comment)).

In the meantime however, there is much to do in low-Earth orbit (in particular for launch, where the Principia flight plan does not exist and third-body effects are irrelevant).

There, the usual analytic perturbation models are adequate; launching to given 𝐽₂-mean elements would be vastly more useful than having the guidance pretend that the osculating elements are true.

In order to use these, the geopotential coefficients must be obtained from Principia.

An API along the lines of

Status ExternalGetGeopotentialCoefficient(int body_index,
                                          int degree,
                                          int order,
                                          out XY coefficient);

should do the job (where the XY contains x = 𝐶_𝑛𝑚, y = 𝑆_𝑛𝑚).

[eggrobin]

We should probably provide the reference radius as well.

GM should be consistent with KSP's, or if it is not, we should enforce that; having an external API to query our gravitational parameter does not sound very useful.