Calculate and store temperature ramp rates #14

It's hard to figure out a way to do this in a generic fashion, but it's probably a good idea to start storing the temperature rates of change automatically because doing it in Grafana works only if you don't do any groupby time() stuff. Observe the following two plots, exactly the same except one of them is using the groupby time($__interval) option in the query:

Screen Shot 2020年05月04日 at 11 28 25 PM

Screen Shot 2020年05月04日 at 11 29 00 PM

The groupby() query is getting hung up on the query interval, and I can't really get around that that the grafana level from what I've seen so far.