copy gpdfit from posterior

[avehtari]

since gpdfit is not exported from posterior, copy the latest version from posterior

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 92.76%. Comparing base (cd6715d) to head (253746b). :warning: Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   92.76%   92.76%   -0.01%     
==========================================
  Files          31       31              
  Lines        2986     2985       -1     
==========================================
- Hits         2770     2769       -1     
  Misses        216      216              

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