v2: Don't merge tables when creating `Problem`

[dweindl]

PEtab allows spreading conditions/observables/measurements/... across multiple tables. So far, the different tables of a certain type are merged when creating a Problem. This is convenient for simulation, but pretty inconvenient when loading/modifying/saving the problem, where one usually wants to maintain the old structure.

With the current object model, it should be relatively easy to store the individual tables separately in petab.v2.Problem.
E.g., change Problem.measurement_table from MeasurementTable to list[MeasurementTable] table and introduce a Problem.measurements property that combines them on demand.

[dilpath]

Could keep measurement_df from petab.v1 for the combined table, and then a new problem.measurement_tables for the original tables. But then problem.measurement_df should be read-only, unless there is a nice way to edit the original tables via the combined table.