Add InteractionsModel for systematic model training an serialization

[rfl-urbaniak]

Introduce InteractionsModel class and a test thereof. Objects in this class:

• load and prepare appropriate data for inference (still to be revised once the new transformation method lands)
• train the interactions model on a given combination of data and forward shift
• serialize and save the resulting guide

A example of usage and downstream functioning is in the updated version of interaction_cities_prototype.ipynb.