Cross Validation: Create a cross validation module

JIRA: PIVOTAL-40975391

Created a new module for applying cross validation on regression
methods. The current interface is setup to work with linear and
logistic regression, as well as ridge, lasso, and elastic net
regression techniques that are upcoming. All other modules can be
supported by adding wrapper functions to work with the cross
validation interface
General Cross-validation: update for GPDB40, 41 and PG84

Solved some compatibility issues
Module Name: General Cross-validation

Fixed a typo
Linear Regression: Add grouping support and heterskedasticity

PIVOTAL: 40576747

Added new api for linear regression along with support for grouping
columns. Also added computation of heteroskedasticity values as an
aggregate.