Fix JaxObjective

[GiacomoFabrini]

• Add pre_post_processor.setter
• Add base_objective wrapper functions (value / value and grad) to JaxObjective

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 68.99%. Comparing base (d8333bd) to head (f883f01).

Files	Patch %	Lines
pypesto/objective/jax/base.py	90.90%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1400       +/-   ##
===========================================
- Coverage   84.48%   68.99%   -15.50%     
===========================================
  Files         157      157               
  Lines       12917    12927       +10     
===========================================
- Hits        10913     8919     -1994     
- Misses       2004     4008     +2004     

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