Add partials support for custom accelerations in tudatpy

[minervino96]

The issue is to add a link between the C++ state estimation code and tudatpy that would allow, for each custom acceleration written directly in Python, to:

• provide no partials (default)
• provide the associated analytical partials
• provide the associated numerical partials
• provide the partial with respect to different parameters

[DominicDirkx]

The first version of this is implemented here:

https://github.com/tudat-team/tudat/tree/feature/custom_acceleration_partials

What is implemented:

• provide no partials (default)
• provide the associated analytical partials w.r.t. translational state
• provide the associated numerical partials w.r.t. translational state
• provide the partial with respect to different parameters

The implemented functionality is (at the moment) only tested for partials w.r.t. position. But, the implementation for position/velocity partials is identical and largely uses the same code. This test will be added soon.

Adding the partials w.r.t. arbitrary parameters will take a little more time/testing, but the framework that's been set up includes these aspects. This framework is, at the moment, limited to use in the custom acceleration model, but written such that it could be extended for use in any acceleration model.

Functionality is not yet exposed to Python. The new interface is limited to adding a single property (newly created settings class) to the estimated parameter for which the custom partial is to be computed. See:

https://github.com/tudat-team/tudat/blob/feature/custom_acceleration_partials/tests/src/astro/orbit_determination/unitTestCustomAccelerationVariationalEquations.cpp#L178
https://github.com/tudat-team/tudat/blob/feature/custom_acceleration_partials/tests/src/astro/orbit_determination/unitTestCustomAccelerationVariationalEquations.cpp#L184

which are the lines in the unit test that apply the numerical and analytical partial of the custom acceleration, respectively