StructureSimilarity is a bit of a mess now. We have different ways of computing the metrics and they don't all have the same features and internal checks :

• Some routines check for residue and atom matching some don't
• Some routines accept multiple chains some don't

We want a unified API for all metrics such as :

def compute_x(self, chain_pairs=None, xzone=None, method='svd', check=True, name=['CA','C','N','O'] ):