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 #1318

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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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