add function for the derivative to StateAgeFunction

[lenaploetzke]

Motivation / Current Behaviour

Currently, the derivation of the StateAgeFunction, which is required in the compute_flow function of the IDE SECIR model, is approximated by a backward difference quotient. Alternatively, it would make sense to build a function eval_derivative into StateAgeFunction in order to be able to decide whether the correct derivation is used or an approximation.

Enhancement description

Implement a function eval_derivative into StateAgeFunction and replace difference quotient in IDE model.

Additional context

No response

Checklist

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