Ordering of groups_covmat

Created by: scarlehoff

While testing #2051, I've noticed that groups_covmat is not following the ordering of the dataset inputs (it is not a problem for #2051 I think). You can check this quickly by simply putting an atlas dataset in the middle of cms. This should not be a problem as long as we are always using it as a dataframe, but I thought it would be good to make you aware of this @RoyStegeman @andreab1997

I'm combing the code to figure out if it is a problem somewhere, and if it is not, I'll explicitly change the ordering of the dataset in the tests so that we are testing the case in which a dataset is in the middle of the others.