Make sure `fit_args` works with non-default esimations in `estimate_effects()`

[Brian Quistorff]

[Zachary S Cacciatore]

Is this what's raising the "ValueError: Need to pass in treatment_period when there are multiple" in the fit method? Tried passing treatment_period **kwarg with indexed ranges in the estimate_effects but still raises same error. For multiple treatment periods this seems like an issue and the documentation is not clear on how to implement this.