AdditionalStateProvider should be available also in non-analytical propagators
Currently, the AdditionalStateProvider interface can be registered only
to analytical propagators
(Kepler, Eckstein-Hechler, TLE SGP4/SDP4, adapter propagator, ephemeris).
Integration-based propagators like numerical and DSST handle additional
states only by integrating
by themselves differential equations provided by the user.
If users want to provide additional states directly to an
integration-based propagator without
having a differential equations, they cannot do it (or they need to set up a dummy equation with
0 derivative which is awkward).
The handling of AdditionalStateProvider should be moved upward in the propagator hierarchy.
(from redmine: issue id 133, created on 2013-04-08, closed on 2013-04-08)