spring cleaning

[jeff-regier]

Refactoring and simplifying the code base in preparation for summer

closes #687 by removing the GalaxyDecoder

closes #697, not by adding a StarEncoder class, but by removing the GalaxyEncoder -- there's just one encoder now.

closes #690 and #691 by removing support for more than one detection per tile -- overlapping tiles are a cleaner way to deal with multiple nearby objects, if we need this functionality

closes #696 because the galaxy parameters no longer depend on the location parameters in the variational distribution, as a consequence of merging the various encoders into a single encoder.

closes #698 by having variational_mode take the output of encoder_batch as its input

closes #715 (I think) through the introduction of a render_images function, which renders full batches of images (Note that FullCatalogDecoder is now named ImageDecoder, and the former ImageDecoder has been removed)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@06d03d5). Click here to learn what that means.
The diff coverage is 79.11%.

@@            Coverage Diff            @@
##             master     #720   +/-   ##
=========================================
  Coverage          ?   70.70%           
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  Files             ?       13           
  Lines             ?     1379           
  Branches          ?        0           
=========================================
  Hits              ?      975           
  Misses            ?      404           
  Partials          ?        0           

Flag	Coverage Δ
unittests	70.70% <79.11%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
bliss/metrics.py	53.73% <ø> (ø)
bliss/surveys/decals.py	0.00% <0.00%> (ø)
bliss/surveys/sdss.py	64.12% <10.20%> (ø)
bliss/train.py	61.90% <57.14%> (ø)
bliss/catalog.py	71.49% <60.00%> (ø)
bliss/plotting.py	43.33% <96.77%> (ø)
bliss/encoder.py	97.01% <96.92%> (ø)
bliss/simulator/decoder.py	99.22% <99.22%> (ø)
bliss/predict.py	100.00% <100.00%> (ø)
bliss/simulator/background.py	57.57% <100.00%> (ø)
... and 3 more

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