OVL feature importance

NOTE: this merge request is based on !87 (merged) and should not be accepted until !87 (merged) is merged. Otherwise, it will pick up all the stuff in !87 (merged) along with the changes enumerated here.

This implements a the feature importance plots and tables for OVL. I ended up changing the syntax for a few function calls and the like compared to what is in !87 (merged). Perhaps the most notable is that I added feature_importance_figure and feature_importance_table as methods of SupervisedClassifier instead of just methods for ClassifierModel. For OVL, it's a straightforward delegation except for the fact that Vetolist.feature_importance_figure also wants things like the time and significance column names, which are stored within the OVL object instead of the Vetolist object (thus the change).

Beyond that, this should be pretty contained.