Add diyps IOB decay

[scottleibrand]

In support of #17 etc., we should add the DIYPS IOB curve formula. For 3h DIA, it uses a bi-linear activity curve that goes from 0 to peak at 75m, then back down to 0 at 3h. The resulting IOB curve is curvilinear, and looks a lot like the Walsh and Medtronic curves. Those curves can then also be scaled to all other DIAs just by scaling the X axis.

https://github.com/openaps/oref0/blob/master/lib/iob/calculate.js
https://github.com/nightscout/cgm-remote-monitor/blob/master/lib/plugins/iob.js