`gather_rms_data` is wrong for system with cofactors since both ligand and cofactors get assigned resname `UNK`

[hannahbaumann]

The gather_rms_data function that is currently used by the hybrid topology protocol by default selects the ligand based on residue name UNK. If a system has cofactors, they also have residue name UNK, resulting in wrong ligand RMSD values.