per-atom PAE scores

[aozalevsky]

models.qa_metric.LocalPairwise is defined on the per-residue level, but as @arogozhnikov explained in chaidiscovery/chai-lab#52, they estimate per-atom PAEs for ligands and non-canonical amino acids/nucleotides (atom vs residue (for canonical polymers) and atom vs atom for ligands and non-canonical residues). It would be nice if we could extend the qa_metric.LocalPairwise definition.

[benmwebb]

This would first need to be added to the dictionary at https://github.com/ihmwg/ModelCIF, then of course we can add support here.

[aozalevsky]

Re-created in ihmwg/ModelCIF#20, but let's keep this one open too because this one will definitely require library support.