Expose ColliderSet::{take_modified, take_removed}

[Ralith]

These are practically necessary when working without a PhysicsPipeline.

A larger change could avoid exposing ModifiedObjects, but it seems like it might be useful as well.

Alternatively, BroadPhaseBvh::update could be modified to pull this data from the ColliderSet itself. In that case the public interface could be narrowed to a method on ColliderSet to clear the modified/removed sets, but this would make it impossible for a user to inject their own modified/removed handles. I'm not sure if that's useful.

[sebcrozet]

Thank you! I don’t love that change, as it increases the risks of user errors, but I don’t have a better alternative yet. Let’s use that until the whole "use BroadPhase independently from PhysicsPipeline" story gets better.