document whether n-body gauge was used

[lgarrison]

According to our documentation, we used Willem Elber's N-body gauge CLASS fork (https://github.com/abacusorg/class_public/tree/6cf8e384da5343277692fdb67df2ee417d23482f), but a source comment suggests the option may have not been turned on?

AbacusSummit/Cosmologies/abacus_base.pre

Line 523 in 3fb493a

# DJE TODO: This sounds like it might be useful, but it crashes on this version

We should re-run CLASS and compare the outputs so we can document what was done.

[lgarrison]

Also tagging @deisenstein in case he remembers anything about what we actually ran