v2: Basic support for multiple models

[dweindl]

Related to #392.

• Let v2.Problem have a list of models
• Support constructing v2.Problem from files with multiple models
• Move some validators to Annotated
• Add some TODOs.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.39%. Comparing base (5e26769) to head (250d87c).

Files with missing lines	Patch %	Lines
petab/v2/core.py	76.92%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   74.40%   74.39%   -0.01%     
==========================================
  Files          61       61              
  Lines        6692     6691       -1     
  Branches     1182     1181       -1     
==========================================
- Hits         4979     4978       -1     
- Misses       1256     1257       +1     
+ Partials      457      456       -1     

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[m-philipps]

Thank you, nothing important to add from my side.

[m-philipps]

Perhaps change the name to make it clear that this modifies the Problem, e.g. set_model?

[dweindl]

model is a property. This function here is usually invoked via problem.model = some_model, it cannot be invoked as problem.model(some_model). I think the assignment should make it clear that this modifies the object.