Copter/Scripting: add get_circle_radius and set_circle_rate

[rmackay9]

This allows scripts to read the radius and write the rotation rate of a copter in Circle mode (or executing LOITER_TURNS). This has been tested in SITL and resolves a 4.1 issue reported by Arace.

[IamPete1]

I think I would prefer to see this done as a direct singleton to AC_Circle so we don't have to clutter AP_Vehicle with bindings that are only used one one vehicle. It also means we can #define out the whole singleton on vehicles that are not copter, we don't currently support per-method defines. A sort of precedent for this is the quadplane bindings that go direct with a specially added quadplane singleton.

[rmackay9]

@IamPete1, thanks very much for the review. I also thought about a direct binding to AC_Circle but after some thought I don't think it's a good idea. The reason is just that it makes it vehicle specific but also more importantly I think it is a layering violation and is inconsistent with how we control the vehicle in Guided mode.