Adding in step and absexp priors of dust and dist

[karllark]

The visualizations of the Scylla and SMIDGE results have shown that the existing prior model in BEAST is not sufficient. Especially when considering the physicsmodel needed for the MegaBEAST (=BEAST prior model). Specifically, the distribution of dust and stars in distance was not modeled as anything other than flat. The stars are generally centered in the plane of a galaxy, often modeled as falling off with some exponential scale height. The is usually seen as concentrated into dust clouds that are located at a specific distance.

Two new prior models are now included "step" (for Av, Rv, fA) and "absexponential" (for dist) to make the priors more realistic.

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (bcf2f08) 42.30% compared to head (794901f) 42.44%.

❗ Current head 794901f differs from pull request most recent head 3bb544f. Consider uploading reports for the commit 3bb544f to get more accurate results

Files	Patch %	Lines
beast/physicsmodel/creategrid.py	69.23%	4 Missing ⚠️
beast/physicsmodel/priormodel.py	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #795      +/-   ##
==========================================
+ Coverage   42.30%   42.44%   +0.14%     
==========================================
  Files         107      108       +1     
  Lines       10096    10132      +36     
==========================================
+ Hits         4271     4301      +30     
- Misses       5825     5831       +6     

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[karllark]

Codacy analysis failure is for a complexity issue. This complexity is needed, so will ignore.