pre-trained encoder that works for arbitrary DES tiles

[jeff-regier]

The tiles are available here.

[zhixiangteoh]

DES tiles are coadded images; so some major changes are required here to first:

• either "repackage"/"redefine" BLISS' DarkEnergySurvey class as DECam (or something along these lines), and then have a new survey class that represents a dataset of DES coadded tiles; or
• enable DES to have two real-image formats: coadded tiles or single-exposure CCDs

since currently BLISS' DES class is really just an accessory to DECaLS essentially as a dataset of single-exposure CCDs.

After which, references to the Y3 catalogs and DES papers might be helpful to set better prior parameters (for CatalogPrior); and then also perhaps enable BLISS to use Y3 catalogs for prediction on DES.