QL: make SKYRBAND meaningful

[sbailey]

Testing http://desi-1.kpno.noao.edu:9090 using code from desispec #702 (now tag 0.25.0) and qlf #253, mostly looking at Observing History window and drill-downs.

SKYRBAND is exactly 100.00 for every science exposure, which smells like a placeholder. Make that an actual meaningful measurement.

@rkehoe

[rkehoe]

@sbailey. Sure. We did not have a calibration vector for flux calibration, but PR #707 now has that. We will have an update soon.

[Srheft]

@sbailey : this QA remained a placeholder for a long time for a few needed info on the input for the QA that were still in the air:

• What is the reasonable range for sky flux reported by the sky monitor? is it going to be written in the header of the raw exposure? if yes, what is the associated header key name? The REF value and ranges are completely made up because I didn't have a clear sense of what are the sensible numbers to use. Could you ping any one who would provide this info?

• What should be reported for science exposures in b and z arms? this QA would only make the comparison between the total flux of the sky fibers and the sky flux reported by the sky monitor in r-band.

[rkehoe]

@sbailey. Just an update on this Issue, once we have the final flux calibration vector from @julienguy we can finish this QA, either as part of the current PR #721 or a subsequent PR.