function of set_new_abundances control?

Nobody has claimed this yet.

In trying to help a user, I've come across a subsection of
!! composition controls
in `controls.defaults` where one can specify a special list of z fractions. However, these appear to be tied to a flag `set_new_abundances`, which does not appear anywhere else in the *.defaults documentation. It is also not understood as a valid `inlist` parameter.

This is tied to another issue I've been raising on and off for a while, which is the conflict between `&controls` settings for `initial_Y`, `initial_Z`, etc., and the `set_uniform_initial_composition` and related set of parameters available in `&star_job`

When to use what (and why) is not clear to users, and even I do not understand which takes precedent when there is a conflict between assignments or why it's necessary to have both. Does anyone know?

Start with the composition controls subsection in `controls.defaults` and compare it with the `&controls` and `&star_job` settings named in the issue. Trace `set_new_abundances`, `initial_Y`, `initial_Z`, and `set_uniform_initial_composition` to determine which settings are valid and how conflicts are resolved. Done means documenting when to use each option and their precedence.

Written by the indexing model from the issue text.