Add age-group reolution to SEIR and SIR Model

[johapau]

Motivation / Current Behaviour

Agegroups are currently not implemented in the SIR and SEIR Models. This leads to some incompatibilities when using model-independent functions.

Enhancement description

Add agegroup templates to the models

Additional context

No response

Checklist

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