Add v2.Problem.has_{map,ml}_objective

[dweindl]

To check for the type of objective function encoded in the PEtab problem.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.46%. Comparing base (a165108) to head (b9e087c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
petab/v2/core.py	64.28%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   75.52%   75.46%   -0.07%     
==========================================
  Files          62       62              
  Lines        6867     6895      +28     
  Branches     1225     1227       +2     
==========================================
+ Hits         5186     5203      +17     
- Misses       1213     1221       +8     
- Partials      468      471       +3     

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[dilpath]

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