Bug if `factorialEstimand` is built from a dataset with categorical treatments

[olivierlabayle]

if factorialEstimand is built on a dataset with categorical treatments, then fluctuation fails.

ex:

dataset[!, treatment] = categorical(dataset[!, treatment])
Ψ = factorialEstimand(
                ATE,
                (treatment,), 
                outcome; 
                dataset=dataset,
                confounders=confounders, 
                outcome_extra_covariates=(),
                positivity_constraint=nothing,
                verbosity=1
            )