Failure when 2 events are close
When using a NumericalPropagator I see various exceptions when two event detectors trigger events separated by less than the detector's tolerance. Here is a failing test case for NumericalPropagatorTest
```java
@Test
public void testCloseEvents() throws PropagationException {
// setup
// create event detectors within the convergence threshold of each
other
EventDetector a = new DateDetector(10, 1, initDate.shiftedBy(1))
.withHandler(new ContinueOnEvent());
EventDetector b = new DateDetector(10, 1, initDate.shiftedBy(1.5))
.withHandler(new ContinueOnEvent());
// action
propagator.addEventDetector(a);
propagator.addEventDetector(b);
propagator.propagate(initDate.shiftedBy(30));
}
```
(from redmine: issue id 234, created on 2016-03-16, closed on 2016-06-29)