RiddersPropagator

This MR aims to use the Ridders algorithm as it is implemented in the covariance_validation_fixture.hpp like a regular Propagator that propagates the covariance in a pure numerical approach. The main goal of this MR is to make it more consistent with the "regular" Propagator interface and therewith allow better debugging (and only this).

• Finish API
• Test its results against current Ridders
• Decide where in the code it will be