Gdr 1848: co-treatment as matrix

[MarcHafner]

Description

What changed?

Related JIRA issue:

Why was it changed?

Checklist for sustainable code base

• [ ] I added tests for any code changed/added
• [ ] I added documentation for any code changed/added
• [ ] I made sure naming of any new functions is self-explanatory and consistent

Logistic checklist

• [ ] Package version bumped
• [ ] Changelog updated

Screenshots (optional)

[MarcHafner]

In the future, we should remove the code for cotreatment as all co-treatment cases are now matrix