[Question]: Grouping covariance matrix with fit_mmrm

### What is your question?

Hi there,

I'm deploying a mmrm using the cov matrix grouping and I would like to do the same within fit_mmrm but I can't find the proper way to pass the grouping parameter. I've tried setting ID to "group/TN_ID" but this lead to an error. I wasn't able to find any documentation about this anywhere, so sorry for the post here on github, it may be the wrong place...

Thanks for clarification and for your great work in general,

Martin

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