Improve Schedulers docs

[Datseris]

• All docstrings have been made "correct": they refer to ordering, and not scheduling, and they refer to the IDs, not the agents
• Deprecated using schedule with the non-default scheduler, as all schedulers are anyways functions of the model, there is no need to pipe them through schedule.
• Highlighted another usage for the schedulers
• Remove wording "once per step" from schedulers: this is clear from the StandardABM docstring. Furthermore, how many times scheduling happens in custom scheduling is unknown.

[Tortar]

I don't think that removing schedule(model[, scheduler]) is a good idea, how do we manage to filter already removed agents from a custom scheduler? This was the motivation for this feature

[Datseris]

ooooooh shit yeah I have completely forgotten about this....

I will revert everything related to removing schedule.

[Datseris]

@Tortar the way we did it before was suboptimal. Now we check inside step! if an agent exists. it is more efficient than wrapping again the scheduled ids in another Iterators.filter. And now, schedule(model, scheduler) is indeed deprecated.

[Datseris]

Alright so I introduced a new function hasid to make this process simpler.

[Datseris]

tests passs locally

[Tortar]

Seems good anyway to me apart from those details!