extend implementations of addToSquad and removeFromSquad

[chdoc]

I have run the in-development autotraining for the better part of a season using this implementation, and I have not seen a single unit get stuck in a generic training session.

[quietust]

The original code is actually slightly different - rather than fetching the squad's global activity and removing the unit from ALL of its events, it should be fetching the unit's position within the squad and then removing the unit from each of the 3 activity/event IDs specified inside:

    auto squad = binsearch_in_vector(df::global::world->squads.all,&df::squad::id,unit->military.squad_id);
    auto position = squad->positions[unit->military.squad_position];
    for (int i = 0; i < 3; i++) { // iterate across possible values of enum "squad_event_type"
        auto activity = binsearch_in_vector(df::global::world->activities.all,&df::activity_entry::id,position->activities[i]);
        if (activity) {
            auto event = binsearch_in_vector(activity->events,&df::activity_event::id,position->events[i]);
            event->removeParticipant(unit->hist_figure_id, unit->id, false);
        }
    }