Model adoption history explicitly

[denised]

The current Excel models were generally created some time in the past. As they get updated over time, there is a need to replace past prognostications about adoption with actual observed adoption. Attempts to coerce the Excel into doing this has led to the largest set of inconsistencies across the models that I am aware of. What we should be doing is modeling adoption history completely separately from adoption prognostication. Then it would be okay that some prognostications are about historical times (because the prognostication was itself made in the past), and we could cleanly model using "observed history" for previous years, and updating it independently of the prognostications themselves.

(This would be an invasive change, but would have the largest code cleanup factor of any single change I have considered.)