Added the simpler interface for cross validation

(1) Added the simpler interface for ridge and lasso
(2) Added lasso cv function, which avoids the limitation of transaction
    locks
(3) Make the interface for ridge and lasso more consistent

More needs to be done:
(1) Documentation for Ridge, Lasso and CV need updates to reflect all
    the changes that have been made
(2) More comments inside the code are needed

Possible improvements:
(1) Move some of the utility functions into Python layer instead of
    plpgsql