Upgrade cohfar_accumbackground

See 4.3 of https://docs.google.com/spreadsheets/d/1lQJzSIE6iUhlRQ5O4gjHBnIpsqFS7w3DQw3wX5_66P8/edit#gid=1881169084

Changes

This MR upgrades the 'cohfar_accumbackground' element of the pipeline (see https://wiki.ligo.org/pub/CBC/Searches/SPIIR/O4bReviewMaterial/SPIIR%20pipeline.pdf).

This repeats all changes from !226 (closed) for these files.

References

1. Review meeting covering Multiratespiir.c, some parts of it may be useful for this review, though it may be quicker to do a new call/respond to comments as needed. (Let me know if you need access): Recording 2024/05/06

Purpose

I intend to split !226 (closed)'s changes into a number of MRs, which won't be functional code versions themselves, but once complete will encompass the full set of changes to upgrade to py3.

This should provide useful code diffs for reviewers.