J2SquaredModel and DSSTJ2SquaredClosedForm are not "implementation friendly"
DSSTJ2SquaredClosedForm
currently returns an empty list for the short periodic terms. It is not confortable for user that want a model for the short periodic terms of the J2-squared model while using the current model for the mean element rates.
The minimum action should be to add in J2SquaredModel
a method providing the short periodic terms. To be "minor release friendly" the default implementation could be to return an empty list.
It would be nice to also add the method updating the short period terms.