Refractoriness always in threshold.py

This implements the automatic addition of "not_refractory" to the threshold condition. It also shows a problem with this approach, though: `PoissonGroup` doesn't use the refractory period but since `not_refractory` is reset in the state updater, it needs a crazy almost empty state updater to reset the `not_refractory` variable at every time step. Obviously we want to handle this better, maybe specific templates for thresholds/state updates without refractoriness.