…tructured _RLearner to be subclass of _OrthoLearner. Small modifiications to first stage and final wrappers in DML and in preprocessing done at the beginning of statsmodels inference.

…ualization inside the _RLearner when both X nad W are None

…learner do not contain all the indices. We now call fit model final only on that subset of indices for which we have calculated their nuisance values.

…learner do not contain all the indices. We now call fit model final only on that subset of indices for which we have calculated their nuisance values.

… classes and models of child classesto omit keyword arguments in their signature if those will always be equal to None.

…abel version.

…riate error messages. Added tests that check that these messages are correctly raised.

… varialbes in the ortho learner

…ession in utilities

…r now until better solution is found. Also creating separate headers for private and public modules on sidebar of docs