reorganized discrete events handling during propagation

src/examples/KeplerianPropagation.java

src/examples/NumericalPropagation.java

src/main/java/org/orekit/attitudes/Attitude.java

src/main/java/org/orekit/forces/ForceModel.java

src/main/java/org/orekit/forces/drag/DragForce.java

.../org/orekit/forces/gravity/CunninghamAttractionModel.java

...va/org/orekit/forces/gravity/DrozinerAttractionModel.java

...n/java/org/orekit/forces/gravity/ThirdBodyAttraction.java

...a/org/orekit/forces/maneuvers/ConstantThrustManeuver.java

...ain/java/org/orekit/forces/maneuvers/ImpulseManeuver.java

...a/org/orekit/forces/radiation/SolarRadiationPressure.java

src/main/java/org/orekit/orbits/Orbit.java

src/main/java/org/orekit/overview.html

src/main/java/org/orekit/propagation/AbstractPropagator.java

src/main/java/org/orekit/propagation/Propagator.java

src/main/java/org/orekit/propagation/SpacecraftState.java

.../java/org/orekit/propagation/events/AbstractDetector.java

...a/org/orekit/propagation/events/AdaptedEventDetector.java

...in/java/org/orekit/propagation/events/ApsideDetector.java

...it/propagation/events/CombinedEventsDetectorsManager.java