Support ipopt with gradient approximation

[MerktSimon]

Ipopt supports approximation of Jacobian/Gradients. With this PR pyPESTO will support it, too (fixes #1284).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.04878% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 53.31%. Comparing base (312f43a) to head (d555910).

Files	Patch %	Lines
pypesto/optimize/optimizer.py	78.04%	9 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1310       +/-   ##
============================================
- Coverage    83.14%   53.31%   -29.83%     
============================================
  Files          153      153               
  Lines        12451    12459        +8     
============================================
- Hits         10352     6643     -3709     
- Misses        2099     5816     +3717     

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[PaulJonasJost]

Thanks, as discussed would probably good to have a small test for this.

[MerktSimon]

just added a test. Not sure what the failing julia test has to do with it.

[PaulJonasJost]

Looks good, thanks :)