Make internal use of `keys` and `names` consistent

[dpanici]

For a future PR: The variable we call keys here and in the other util functions below is the same as what we call names in the compute functions above and in other files. We should probably rename for consistency across the code.

Originally posted by @ddudt in https://github.com/PlasmaControl/DESC/pull/1027#discussion_r1669324227