Fix unwanted amici model recompilation

[m-philipps]

changes:

• PetabImporter checks for existing amici model in the location specified by output_folder before re-compiling.
• closes PetabImporter does not check for existing amici models in location specified by output_folder arg. #1318

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.44%. Comparing base (ee852cb) to head (6c92396).

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1319   +/-   ##
========================================
  Coverage    84.44%   84.44%           
========================================
  Files          153      153           
  Lines        12456    12455    -1     
========================================
  Hits         10518    10518           
+ Misses        1938     1937    -1     

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[dweindl]

👍 thx