Move PeakLikehoodFlux to meas_base framework
Perry did this work on tickets/DM-976 in an effort to get it me quickly; I'm cherry-picking the relevant commits to u/jbosch/DM-446 to keep the git history synced up with the issues.
Everything has now been moved to u/jbosch/DM-446, reviewed, and cleaned up to reflect the review. Will merge as soon as doing so will not impact stack release efforts.
Merged to master.