DESI average throughput from SV1 and SV3

[julienguy]

Use the new measurement of the throughput presented in https://desi.lbl.gov/DocDB/cgi-bin/private/ShowDocument?docid=6157 .

• define new attributes to the class desispec.averagefluxcalibration.AverageFluxCalib (median_seeing, median_ffracflux, fac_wave_power, ffracflux_wave,first_night)
• use this when computing the sky magnitudes
  (a new version of the average flux calibration following this new data model is saved in the spectro calibration svn repo, see https://desi.lbl.gov/trac/browser/calib/desi_spectro_calib/trunk/spec/fluxcalib)

[coveralls]

Coverage decreased (-0.02%) to 27.603% when pulling f670268 on throughput-sv13 into 040a4d5 on master.

[julienguy]

merging this PR that affects only the sky magnitude after the improved measurement of the throughput (3% change in calibrated sky flux).