Interfaces for ODE integrators

Hello,

I thought we could create one interface for each integrator scheme e.g. RK4, that would allow to avoid some code duplications between Field and standard implementations (and possibly state interpolators, TBC), like for the coefficients behind getA, getC, etc. It would also centralize where names are defined, facilitating their exposure in light of this Orekit issue. Thoughts?

Best,
Romain.