Draft: Fix retrograde spin treatment
Fixes #22 (closed) by using Jhat dot Lhat to consistently flip sign both for the final spin and for the post-merger extension of the Euler angles, for retrograde-spin cases.
Pending to be merged after rerunning standard sanity checks (e.g. as documented here https://git.ligo.org/waveforms/reviews/seobnrv5/-/wikis/tests_bug_fix_15052023) and mismatches with NR for configurations that are affected by the change.