rename math variables

[jeff-regier]

This PR closes #330 by renaming many variables that are frequently used throughout Celeste. Originally, these variables' names matched the mathematical notation. Now it makes more sense to use descriptive variable names.

@andymiller Please rebase once this is committed--could be a lot of merge conflicts otherwise.

[kbarbary]

Can we make constants UPPER_CASE? It's pretty conventional. e.g. https://docs.julialang.org/en/latest/stdlib/constants/#

[codecov-io]

Codecov Report

Merging #682 into master will decrease coverage by <.01%. The diff coverage is 86.1%.

@@            Coverage Diff             @@
##           master     #682      +/-   ##
==========================================
- Coverage   68.84%   68.83%   -0.01%     
==========================================
  Files          37       37              
  Lines        4160     4159       -1     
==========================================
- Hits         2864     2863       -1     
  Misses       1296     1296

Impacted Files	Coverage Δ
src/deterministic_vi/ConstraintTransforms.jl	100% <ø> (ø)	:arrow_up:
src/model/psf_model.jl	100% <ø> (ø)	:arrow_up:
src/SensitiveFloats.jl	93.24% <ø> (ø)	:arrow_up:
src/SEP.jl	79.38% <ø> (ø)	:arrow_up:
src/ArgumentParse.jl	89.47% <ø> (ø)	:arrow_up:
src/Configs.jl	100% <ø> (ø)	:arrow_up:
src/model/image_model.jl	100% <ø> (ø)	:arrow_up:
src/model/imaged_sources.jl	100% <ø> (ø)	:arrow_up:
src/multinode_run.jl	0% <ø> (ø)	:arrow_up:
src/deterministic_vi/elbo_kl.jl	51.19% <0%> (ø)	:arrow_up:
... and 22 more

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[jeff-regier]

@kbarbary I've made all the global constants that this PR changes upper case.

[kbarbary]

I can't claim to have read every line in detail, but this looks good to me: a big improvement in readability.