decompose graph.curve.params()?

[d-morrison]

graph.curve.params() consists of two major sections:

1. computing quantile data for the longitudinal curves, and
2. graphing those quantiles.

For readability, we should probably decompose those steps into subfunctions.

Previously, we considered using a fortify() method to encapsulate step 1. However, the fortify() generic now appears to be headed toward deprecation: https://ggplot2.tidyverse.org/reference/fortify.html

It looks like the intended replacement is generics::augment(). Let's use that instead?