Indicating available / applicable arms

[leandrolanzieri]

Hello! Thanks for this nice project!

I'm currently integrating mabwiser into my project. I have the situation that in given contexts certain arms are not applicable and shouldn't be considered in the prediction. This doesn't mean that I want to _drop_existing_arm, just ignore it. I'd like to keep the models for each arm.

Is there a way this could be achieved with the current API?

Thanks!