univariate preselection based on RMSE, added new unit testing

[sborms]

Model_building - univariate_selection

Model_building - univariate_selection

Changes made

• added model_type parameter to compute_univariate_preselection() function
• added preselect_rmse_threshold parameter to compute_univariate_preselection() function (such that preselect_auc_threshold parameter remains, to avoid backward compatibility API clashes)
• separated compute_univariate_preselection() function into calculation for AUC (classification) and RMSE (regression)
• changed get_preselected_predictors() function such that it picks predictors ordered based on metric computed before
• modified documentation to indicate computation of AUC or RMSE depending on input
• added new unit tests for univariate selection (tests\model_building\test_univariate_selection.py)

Linked issues

Resolves #68

[sandervh14]

Looks good!! (Checked high-level. If there would be remaining issues, we'll notice and fix when testing on the regression MVP case.)