Document the Estimator Object from the numerical_simulation module

[jo11he]

I noticed that I stopped my API documentation work at the level of the estimation_setup and estimation modules.

The Estimator object, being the highest-level and arguably most important part of any tudatpy estimation task, is located a level above, which is why it slipped my attention. I'll add that to the API docs asap.

[jo11he]

Done. While I was at it I also added the docs for the VariationalSolver.
Since this is very high-level stuff, I think it is a good idea to review @DominicDirkx.
https://tudatpy.readthedocs.io/en/latest/numerical_simulation.html#classes

[DominicDirkx]

Great! I'll review it in the coming weeks