LogNormal DFE for humans

[clararehmann]

addresses #1484

I used table S3 from Huber et al. 2017 for the parameters and refactored the mu value of the LogNormal distribution to account for DaDi vs. SLiM scaling. I also modified the DFE tests so that they can accept LogNormal distribution arguments (which are strings, not floats).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (2ccbf88) to head (2fa7069).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1698   +/-   ##
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  Coverage   99.85%   99.85%           
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  Files         139      139           
  Lines        4799     4810   +11     
  Branches      470      470           
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+ Hits         4792     4803   +11     
  Misses          3        3           
  Partials        4        4           

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[petrelharp]

Excellent!