Issue #75 PIGs for linear regression

[sandervh14]

Story Title

Evaluation - pigs_tables

Changes made

• adapted PIG table & plot creation to support regression targets additionally
• did a minor(!) expansion to the unit tests, the other tests I left in comments really require exposing a variable (df_plot) internal to the function + checking matplotlib's plot and ax object internals. Trying out the PIG plotting on our MVP regression case will show us immediately though if there is a problem left or not, anyway.

How does the solution address the problem

This PR will roll out PIGs for regression too.

Linked issues

Resolves #75.

[sborms]

Looks good. Smart that you rename the variables internally so it is clearer.