Soothsayer created from already fitted models

[JSzitas]

It is quite obnoxious having to retrain many individual models (if using soothsayer for model combining/selection)

A quick sketch of an interface around this:

Take a mdl_df object, and add a new column for soothsayer by calling some function, i.e.

add_soothsayer( mdl_df, soothsayer_model, ... )

and potentially have it look for models it would be missing (and compute those) or have it take the fitted models (and combine them).

[JSzitas]

Resolved as of ecbc77a0022d3f921e5f82b83ea72c106ea88068