Fixing the grouping and the thcovmat flag

Created by: andreab1997

The grouping used in the thcovmat fit is broken because groups_dataset_inputs_loaded_cd_with_cuts does not use group_dataset_inputs_by_fitting_group and therefore is not grouped as the thcovmat (even if the groupings are similar so sometimes it happens to be correct by chance).

Moreover, the thcovmat group is used also when both use_thcovmat_in_sampling and use_thcovmat_in_fitting are false.