Clarify PEtab problems with multiple models

[dweindl]

So far, the spec says:

Parameter estimation problems can comprise multiple models. For now, PEtab allows to specify multiple SBML models with corresponding condition and measurement tables, and one joint parameter table. This means that the parameter namespace is global. Therefore, parameters with the same ID in different models will be considered identical.

I don't think this is sufficient to implement support for that.

Clarify, for example:

• Is there is a global namespace for conditions/experiments/... across that problems list in the yaml file?
• Can a measurement file from problems[0] refer to observables/experiments from problems[1], and so on. If so, how to specify to which model an observable / a measurement belongs?
• ...