MMCIF utilities

[AntonOresten]

Adds some utility functions that I converged to while extracting MMCIF data for dataset prep. Not sure if it suits this package since MMCIF dictionaries are very large and they haven't been exposed to the API yet.

However, I think this is the most straight-forward way of e.g. going from the author-assigned chain IDs to taxonomic IDs:

julia> mapmmcif(mmcifdict,
           "_atom_site.auth_asym_id"   => "_atom_site.label_entity_id",
           "_entity_src_gen.entity_id" => "_entity_src_gen.pdbx_gene_src_ncbi_taxonomy_id")

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