Pluggable solvers #2
Description
It has been discussed by others that it would be a nice feature to provide pluggable solvers for Keplerian orbits. I think this is easily achievable by defining a kepler_solver function which uses a KeplerianSolver type hierarchy along with a global default. I think this would be a good opportunity to drop the dependence on AstroLib.jl and move the relevant code into this package.