Rearrange exercise to have function creation be at the locations they are needed

[lwjohnst86]

This exercise in Multiple Models section:

## Exercise: Creating functions for model results and adding as a target in the pipeline

should probably be split so that part 2 (the calculate_estimates()) should be moved up to start in the place that we convert the function. It was a bit confusing to go back to the code they created several chunks above and make a function from that.