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• Switch from no longer maintained pyjulia to juliacall package (pyjulia is no longer maintained #1397)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]

Codecov Report

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

Project coverage is 83.21%. Comparing base (18be6e3) to head (7ef409f).

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

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1398      +/-   ##
===========================================
- Coverage    84.50%   83.21%   -1.29%     
===========================================
  Files          157      157              
  Lines        12955    12955              
===========================================
- Hits         10947    10781     -166     
- Misses        2008     2174     +166     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sebapersson]

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

[m-philipps]

No, so far only the petab objective.