Add information about fibers with low transmission

[julienguy]

Fibers with low transmission (<70% of average) should not be assigned to targets (because they will on average be assigned a LOWEFFTIME fiber status bit during the spectroscopic pipeline exposure QA) and fibers with even lower transmission (<50%) should not be counted as valid sky fibers.

We need a mechanism to save this information in DESIMODEL in order to pass it to fiber assignment. This information has to be time-dependent (for example the transmission of 18 fibers changed after the positioners have been replaced during the summer 2021 shutdown, see https://desi.lbl.gov/DocDB/cgi-bin/private/ShowDocument?docid=6460).