feat: fuller functionality for models_ attribute

[TheLemonPig]

The models_ attribute added to the BMS (and soon DARTS) regressors was intended initially only for the dissimilarity sampler. As a result, there is no functionality around this attribute built into the theorist.

Proposed additions:

1. add parameter a num_models to present_results() which allows the user to determine how many models they would like to see results for (i.e. function string and plot of prediction and data)
2. add description length or alternative scoring result to present_results()

All additions to functionality should be added to the unit test