v2: Handle multi-model problems

PEtab v2 clarifies the semantics for using multiple models inside a single PEtab problem, see https://github.com/PEtab-dev/PEtab/pull/622/files. We need to adapt to that.

This requires:

• #391
• Multiple models inside Problem, or adding a CompositeProblem on top of Problem
• Updating many validation rules
• petab.v2.C.MODEL_ID
• ...

Start by reviewing the PEtab v2 semantics in the linked pull request #622 and the completed prerequisite #391. Then investigate how Problem currently represents models, whether a CompositeProblem is needed, and the validation rules affected. Done means supporting multiple models, updating the validations, and adding petab.v2.C.MODEL_ID.