PartialDerivativesEquations does not work with non-Cartesian elements
Using PDE with the propagator's orbitType set to a non-Cartesian value does not produce meaningful results. See the discussion on the dev list: https://www.orekit.org/wws/arc/orekit-developers/2017-08/msg00010.html
One workaround is to compute the STM in Cartesian elements and then
transform it to the desired element set using equation 7.32 in
Montenbruck & Gill:
```
STM (Keplerian) = d(kep)/d(cart) * STM (Cartesian) * ( d(kep)/d(cart)
)^-1
```
Another workaround is to use a FieldNumericalPropagator with the orbitType set to the desired value, and then extract the STM from the final orbital elements.
(from redmine: issue id 351, created on 2017-08-11)
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