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
Utilities: Fix issue with keyerror
Estimator methods: Replace verbose since it is keyword
Estimator methods: Change verbose name in python as well
NULL handling for estimators of Linear/Logistic regressions

Pivotal Tacker: 57195098 and 57195034

Gracefully handle the NULL values for Huber-White/Clustered/Marginal on
both linear and logistic regressions.
Estimators: PEP8 certification + remove verbose keyword

Changes:
    - Multiple small changes to remove PEP8 errors
    - Ensure that 'verbose' is replaced  'verbose_mode' due to possible keyword
    error in Postgresql
    - Re-added use of 'regr_coef' in marginal code to not call underlying
    multinomial regression if the coefficient is provided.
Turn off ORCA in lda.get_perplexity and simplify the query
Merge branch 'integration' of https://github.com/madlib-internal/madlib into integration