adjust naming and assignment of ABM age groups

[reneSchm]

Motivation / Current Behaviour

We want to apply recent changes to age groups in abm_minimal.cpp to all other ABM files.

Enhancement description

The proposed changes include

• renaming age group variables to use lower-case letters, so they do not look like macros,
• changing value assignment from AgeGroup(num_age_groups - n) to simply AgeGroup(n).
  • The idea for the former was that it could help a user assigning AgeGroups values in [0, num_age_groups). This is better achieved using documentation and some asserts in the ABM code.

Additional context

No response

Checklist

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

[khoanguyen-dev]

Maybe we should also include a check for all functions that use AgeGroup that the given variable within [0, num_age_groups).