Mock up offline SNR updates

[dkirkby]

The desisurvey.scheduler.Scheduler object owns the SNR2FRAC array used to make next-tile decisions and initialize ETC integration during a night. During the night, SNR2FRAC values are updated from ETC estimates, and can be save / restored from a FITS file.

The sims do not yet have a concept of the independent SNR analyses performed by QL and the offline pipeline, so this issue is to mock something up that allow us to study the impact of systematic and statistical (offline - online) differences, as well as delays in the availability of offline updates (e.g., due to network or NERSC outages).