TSNR camera coadd fix

[michaelJwilson]

Addresses #1193. Runs through the use case example provided there and passes other vetting on values.

For future consideration: rename TSNR_QSO to TSNR_QSO_BRZ etc. to mirror nomenclature for flux etc.

[coveralls]

Coverage decreased (-0.005%) to 28.902% when pulling 20fd290 on tsnr-camera-coadd-fix into f3895a1 on master.

[sbailey]

Thanks for the fix, looks good. I'm open to discussing TSNR2_XYZ vs. TSNR2_XYZ_BRZ but let's merge this bugfix now without changing the datamodel, and independently decide if we want to change the column name (and let's decide that before running Denali, i.e. soon).