Update julia objective

[m-philipps]

• Switch from no longer maintained pyjulia to juliacall package (pyjulia is no longer maintained #1397)
• Update example; switch to using ForwardDiff

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.54%. Comparing base (47ae5d5) to head (d0f7ff0).

Files with missing lines	Patch %	Lines
pypesto/objective/julia/base.py	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1398      +/-   ##
===========================================
- Coverage    82.74%   81.54%   -1.21%     
===========================================
  Files          163      163              
  Lines        13892    13892              
===========================================
- Hits         11495    11328     -167     
- Misses        2397     2564     +167     

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[sebapersson]

Does this PR also account for the interface to PEtab.jl?

[m-philipps]

No, so far only the petab objective.