Regress: Bug fix for logistic regression, many small changes for robust
variance.

The commit brings the robust variance code in line with the code review.
In addition, the logistic regression will now throw an error if the
output table name parameter is a name for an already existing table.
Unify the interface for robust multinomial and revise documentation

Pivotal ID: 52350975
Regress: The usage/help function for robust multi-logistic has been
corrected

The previous usage/help was out of date.  This has been corrected.
Utilities: Correct return behaviour for array_to_string

Previously we didn't have a return value for array_to_string in the
utilities leading to issues in GPDB 4.1. Further, marginal effects
also didn't take care of the special handing required for arrays
passed in GPDB 4.1 from SQL to Python. These issues have been rectified
in this commit.
Utilities: Correct return behaviour for array_to_string

Previously we didn't have a return value for array_to_string in the
utilities leading to issues in GPDB 4.1. Further, marginal effects
also didn't take care of the special handing required for arrays
passed in GPDB 4.1 from SQL to Python. These issues have been rectified
in this commit.
Added the interface for marginal effects for multinomial logistic regression.
Started the internals. They won't work right now
Stashing changes for later
Working code for multilogistic regression without standard errors
Working code for standard errors
Changed the way the marginal variables were queried
Reomved a random log. Things look good now
Merge branch 'feature_margins_multilogistic' of https://github.com/madlib-internal/madlib into feature_margins_multilogistic