Change return type of check_constraints function in the Populations class to bool

[lenaploetzke]

Motivation / Current Behaviour

The check_constraints and apply_constraints functions in the Population class are currently not tested.

Enhancement description

If the return type is changed to bool (as in the check_constraints function of all parameter classes), you could easily test them. The check_constraints function of CompartmentalModel could also return a bool.

Additional context

No response

Checklist

• [X] Attached labels, especially loc:: or model:: labels.
• [X] Linked to project