Generic CV: Allow NULL parameter
Common utility functions for regularizations

Some useful functions for Ridge, LASSO and Elastic-net.
Make utils_regularization.py_in compatible with array[] expressions
Make utils_regularization.py_in compatible with array[] expressions
Make utils_regularization.py_in compatible with array[] expressions
Linear Ridge regression together with cross-validation support
Ridge linear regression: Make it accept array[]

Also removed some argument validations that are related to the independent
variable, because the independent column name might be an actual SQL
expression
Elastic Net

will finish IGD, FISTA, CD, BCD optimizers

will add cross-validation support
Elastic Net: Be consistent with previous changes

utilities changed locations, also elastic net changed location. Just
need to make the elastic net files be consistent with all these changes.
Elastic Net: Continue programming
Continue programming FISTA
Continue programming FISTA
Added state definitions for IGD and CD
IGD is over-engineered due to its use of previous old code. Start to
remove all the over-engineered parts.