Breaking change in `propagate!` about satellitetoolbox.jl

[ronisbr]

Hi @ThatcherC

Sorry! I completely missed this bug report. Yes, this was on purpose in v0.9.0.

The propagator API, which is described here https://github.com/JuliaSpace/SatelliteToolbox.jl/blob/master/src/orbit/propagators/PROPAGATORS.md, was changed to return only the state vector (position and velocity) or the osculating elements. I removed the first argument, which was the mean elements, because some propagators cannot obtain this easily. For example, if we keep this, then the numerical propagators I want to implement will need to make an interpolation at each propagation to return the mean elements. To avoid that, the propagation functions must only return now the osculating elements. I also add a function called get_mean_elements that will return the mean elements if you want.