This allows us to detect plug/unplug events immediately, instead of
having to wait for the next poll event.
Unfortunately, capacity changes do not appear to generate events :(
The ramp particle is configured with a tag name and a list of
particles.
At instantiation time, a single particle is selected from the list,
depending on the tag value and it's minimum and maximum values.
I.e. this particle acts kind of like a progress bar.
