Multilogit: NULL handling for regression and estimators

Pivotal Tracker: 57194990, 57195142

Additional author:
    Shengwen Yang <syang@gopivotal.com>

Changes:
    - Added NULL handling for Multinomial logit regression
    - Added NULL handling for robust/clustered/marginal estimators for
    Multinomial logit regression
    - Changed interface of mlogit to output a table instead of returning result
    type
    - Refactoring to improve code flow