An identifier to group structural models into collections or sets.
A cluster of models and its representative can either be grouped together
or can be separate groups in the ma_model_list table. The choice between
the two options should be decided based on how the modeling was carried out
and how the representative was chosen. If the representative is a member of
the ensemble (i.e., best scoring model), then it is recommended that the
representative and the ensemble belong to the same model group. If the
representative is calculated from the ensemble (i.e., centroid), then it is
recommended that the representative be separated into a different group.
If the models do not need to be grouped into collections, then the
_ma_model_list.model_group_id is the same as _ma_model_list.model_id.