Converger interface

Is this a reference to the "schedule" in "schedule_convergence"? If so, it doesn't have any details in it as to how it's called (scheduling); it simply encapsulates that the `IConverger` does not guarantee that calling this method will do something (like a single convergence cycle) *right now*, but at some indeterminate (hopefully soon) point in the future. The first implementation of this right now in otter will probably synchronously do some stuff, but that's an implementation detail.

I originally named this function converge, and then realized it'd probably be confusing with the function converge that actually does some convergence.