A list of settings and values for vtreat regression fitting.
Please see
https://github.com/WinVector/vtreat/blob/main/Examples/fit_transform/fit_transform_api.md,
mkCrossFrameCExperiment,
designTreatmentsC, and
mkCrossFrameNExperiment,
designTreatmentsN,
prepare.treatmentplan for details.
regression_parameters(user_params = NULL)filled out parameter list
list of user overrides.