Add log-logistic distribution

[devmotion]

Updated and improved version of #1454.

Adding reference tests against R is a bit cumbersome currently, see #2007. Edit: I added tests against R nevertheless by locally commenting out all other reference tests (and without environment, ie probably not reproducible).

Closes #1454.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 86.45%. Comparing base (e5d4fef) to head (e209820).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2008      +/-   ##
==========================================
+ Coverage   86.36%   86.45%   +0.08%     
==========================================
  Files         146      147       +1     
  Lines        8788     8844      +56     
==========================================
+ Hits         7590     7646      +56     
  Misses       1198     1198              

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