For some reason I didn't look at the WA2 instrument until afer I
wrote this, so it's different in how it handles 'before/after'
quantities (which I called 'momentary' in the code) - instead of
adding a 'mean' it just adds two separate metrics: one for before and
one for after. Personally I think this is more useful (esp. for
temperature) but if necessary I can revert to the old behaviour.
This also doesn't add the helpful classifier telling you the hwmon
device, because we don't have immediate access to that info from the
devlib Instrument. If necessary I'm sure it could be added later.
Furthermore it doesn't have a parameter to filter the sensors, just
collects from them all.
