Modify compute_eval_from_predictions_test

- Modified this test to use the main() function which calls the compute_eval_from_predictions() function such that the test covers the whole script.
Modify learner.py to include SGDRegressor for model_params

- Since SGDRegressor is a linear model that has weights and intercepts, it should be included in the list of models that get the model_params property.
Expand the regression tests

- Modified the current regression test which only tests LinearRegression to include all other linear models.
- Added separate tests for tree and ensemble regressors .
- All tests now include un-rescaled as well as rescaled versions of the models.