Growth Rate

[dsweber2]

There were a couple of ways this was suggested to be implemented. The most straightforward was just adding it as a predictor; more sophisticated were:

• using a projection based on it
• using it as a threshold for other variables

Will probably want to consult w/@brookslogan and @dajmcdon about which of these to actually implement/how to go about it. Either of the more complicated relies on step_mutate.