Course Inits as start of history

Currently, course initialisations overwrite the learner model for all symbols in the course. This is unfortunate, since they are primarily thought of as a "priming" of a learner model, i.e. when it is still barren. Would be nice to have course inits be the first thing that happens to a blank model and then re-compute the rest of the model from events.

This needs some more thought on what exactly is supposed to happen when multiple courses contain the same symbol, as they inevitably will, but I think starting with all course inits in the order they were given and then following with all other interactions would be a solid approach.