Add functionality to define metrics (i.e. accuracy, precision, recall, f1) to run on the above joined table

[shreyashankar]

We want a library of common ML metrics. For now, we can include the following:

Binary Classification

• accuracy
• precision
• recall
• F1 score
• ROC / AUC

Regression

• mean absolute error
• mean squared error
• mean absolute percentage error
• R2