Weak lensing simulator

[timwhite0]

This PR gets our weak lensing simulator up-to-date with master.

Changes to the weak lensing case study:

• In LensingPrior, shear and convergence are now constant across the images. The current config uses large shear and convergence.
• Two notebooks: One demonstrates the robustness of shear two-point correlation functions to different noise distributions, and the other analytically derives the effects of weak lensing on galaxy shapes.

Changes to the main BLISS decoder:

• We add a helper function to render_galaxy so that we don't have to repeat code in the lensing subclass of the decoder. Note that we can't just do super().render_galaxy() in the lensing decoder because we don't want to convolve the galaxy with the PSF until after shear and convergence are applied.
• We allow ourselves to circumvent sampling the SDSS background in render_image, as the SDSS frame from which the background is sampled by default is smaller than the images we want for our case study.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.17%. Comparing base (a1c981b) to head (c02ee4d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
bliss/simulator/decoder.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1071   +/-   ##
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  Coverage   76.17%   76.17%           
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  Files          22       22           
  Lines        3072     3077    +5     
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+ Hits         2340     2344    +4     
- Misses        732      733    +1     

Flag	Coverage Δ
unittests	76.17% <91.66%> (+<0.01%)	⬆️

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