Difference in behavior between NumericalPropagator.addForceModel() and FieldNumericalPropagator().addForceModel()
Running
```
NumericalPropagator np = ...;
np.addForceModel(new NewtonianAttraction(...));
```
will replace the current central attraction force instead of adding a new force model. On the other hand
```
FieldNumericalPropagator<>fnp = ...;
fnp.addForceModel(new NewtonianAttraction(...));
```
will add an additional copy of the central attraction force, making the satellite orbit around the Earth twice as fast. The code in FNP seems much simpler than the code in NP, so perhaps the best solution is to just add a note of the difference in behavior to the method comment for FNP.addForceModel().
(from redmine: issue id 350, created on 2017-08-11, closed on 2017-11-25)