v2: Handle multi-model problems

[dweindl]

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:

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

[dweindl]

• Multiple models inside Problem, or adding a CompositeProblem on top of Problem

I would currently go for changing petab.v2.Problem.model: Model to petab.v2.Problem.models: list[Model].

We might want to add some API to split a multi-model problem into separate petab.v2.Problems, where each has only a single model and the associated measurements/conditions/observables/... .

[m-philipps]

We might want to add some API to split a multi-model problem into separate petab.v2.Problems, where each has only a single model and the associated measurements/conditions/observables/... .

Sounds good to me