Refactor Generative Prior to use DES sources

[kapnadak]

• Separated generative prior into two independent priors for background and cluster
• Background sources are sampled directly from DES catalogs
• Background prior samples sources 40 times (for 40 images) from a single DES catalog before switching to a new DES catalog. This makes catalog generation almost 40x faster since loading dataframe was a bottleneck
• Allow GalSim to determine the correct stamp size to use

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.41%. Comparing base (6b63898) to head (04e8da0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1031 +/- ## ======================================= Coverage 92.41% 92.41% ======================================= Files 25 25 Lines 3125 3125 ======================================= Hits 2888 2888 Misses 237 237 ``` | [Flag](https://app.codecov.io/gh/prob-ml/bliss/pull/1031/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prob-ml) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/prob-ml/bliss/pull/1031/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prob-ml) | `92.41% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prob-ml#carryforward-flags-in-the-pull-request-comment) to find out more.

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