scheduling between calls to step handlers and events handlers in ODE is wrong

[maisonobe]

When integrating an ODE with discrete events, user code is called back by implementing
step handler and events handler interfaces that are called on the fly by the integrator.

The current scheduling of these calls is that the handler of the event at the end of the
step is called first, then the handler of the step itself is called later on. See Proposal for step handling overhaul in Orekit forum for
a more complete explanation.

The call order for event handlers and step handler should always be consistent with integration direction.