ForceModel should have a field-based init method
The init method for force models only has a primitive double signature, despite almost all other methods in the interface have both a primitive double and a field signature.
This also applies to DSSTForceModel.
This can be fixed in a compatible way using a default implementation that delegates to the primitive double method. Indeed, this is what field-based propagators do: they convert the arguments to primitive double and call the only existing method.