Weak lensing DC2 updates

[timwhite0]

A few recent updates to the weak lensing case study:

• New version of to_toy_catalog in LensingDC2DataModule that circumvents n_sources entirely.
• Reorganized notebooks.
• We now compute average ellipticity per tile as a baseline estimator for shear.
• New strategy for loading in the DC2 catalog. Previously we were using all ~128 million galaxies from CosmoDC2, but we realized that we only want to use the ~10 million from the truth table (more specifically, the truth columns from the object_with_truth_match table).
• Simulator is now up-to-date with master. Currently uses the SDSS-like decoder from master along with a toy lensing prior.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.73%. Comparing base (bc88cfe) to head (881a110).
Report is 1 commits behind head on master.

Files	Patch %	Lines
bliss/encoder/variational_dist.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1061      +/-   ##
==========================================
- Coverage   80.76%   80.73%   -0.04%     
==========================================
  Files          23       23              
  Lines        3068     3073       +5     
==========================================
+ Hits         2478     2481       +3     
- Misses        590      592       +2     

Flag	Coverage Δ
unittests	80.73% <60.00%> (-0.04%)	⬇️

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