Added Ridge and LASSO regulation for linear regression; Cross-validation

(1) Ridge regulation for linear regression
(2) LASSO regulation for linear regression
(3) A general cross-validation function for modules that comply with a
consistent SQL API
(4) Specific cross-validation functions for Ridge and LASSO
(5) More functions in convex module, which can be used by other modules,
    but they are not exposed by any SQL interface.