Separate ODEEventDetector and ODEEventHandler

[maisonobe]

In Hipparchus ODE module, events detection is performed using switching function g. When the sign of this function changes, the event is triggered and eventOccurred is called. Both methods are declared in the ODEEventHandler interface. This has been done this way since years. This is however cumbersome as events detection and events handling cannot be separated.

In Orekit, there are two different interfaces for the same purpose. Users can implement their own event handling and associate it with existing event detectors already provided by the library without needing to rewrite the detectors. This change was made years ago and greatly improved the library. It was really needed because Orekit provides a lot of events detectors. It was not needed up to now in Hipparchus because Hipparchus does not provide any detector by itself.

However, as part of implementing #220, I would like to have a specialized event detector provided by Hipparchus (it would detect the end of the current step), and let users add their own handler for it.

This change intends to reproduce in Hipparchus what has been done for a long time in Orekit: separate events detection and events handling.