Add regression metrics

[joshuawe]

The implementation of regression metrics is still outstanding.

The metrics for classifiers are much more visualization heavy than for regressors. This is unfortunate, many metrics boil down to a simple number. But some metrics can be visualized.

We need the following metrics/plots:

• [ ] MSE over predicted value
• [ ] MAE over predicted value
• [ ] RMSE over predicted value
• [ ] Plot y_pred vs y_true
• [ ] Plot residuals vs y_true, where residuals are (y_pred - y_true)

Check sources:

• https://juanitorduz.github.io/lm_viz/
• https://machinelearningmastery.com/regression-metrics-for-machine-learning/
• https://scikit-learn.org/stable/modules/model_evaluation.html#regression-metrics

[joshuawe]

For residuals vs y_true visualization see: sklearn.metrics.PredictionErrorDisplay