Merged the reformat of multi-logistic regression and its estimators

Pivotal Tracker: 60428124 60053908

Each category now occupies a separate row. This better output format is
implemented for multilogit regression and Huber-White estimator,
clustered estimator, marginal effect.
Marginal Effects: Fix invalid key in python error
Multilogistic: Return error if all observations in category has missing data

Pivotal Tracker: 60643012

Changes:
    - For mlogit and all estimators for mlogit, we find the all categories
    in data and subtract from this set all categories with at least 1 non-missing
    data observation. If this set-difference is not empty, then an error is
    returned.
    - Also fixed a problem in Python AL for validation of arguments.
Merge branch 'integration' of https://github.com/madlib-internal/madlib into integration