Convert `mag6t.ramp_button` and `.switch_heater` to `PVPositioner`

One difficulty in this problem is that in each step we want to wait for certain conditions. For example, in the staging process, we first click on the ramp button, then wait for the supply current to match the magnet current, then turn on the switch heater, then wait for the magnet status to change. Currently this is done in a workaround, using the `SetSignal` that simply returns a subscription status based on an input device and function (Codacy doesn't like `SetSignal`). Then using `abs_set` (with `wait=True`) we can make the `RunEngine` wait for each step to complete.

I suspect the "best" way to do this is by converting `.ramp_button` and `.switch_heater` into `PVPositioner`. I will try to do it later, for now we can just leave it as is.

_Originally posted by @gfabbris in https://github.com/APS-4ID-POLAR/ipython-polar/issues/99#issuecomment-805865176_