Feature request: allowing multiple dictionaries or dictionary extensions in audit_conform

[drlemmus]

Many programs write mmCIF items and categories that are not (yet) part of the mmcif_pdbx dictionary. Some may even want to provide a formal definition of those items/categories but there is no mechanism to include a dictionary extension. Having such an option would allow interoperability without having to wait for the mmcif_pdbx standard to be updated.

I think that one rule has to set though, a dictionary extension should be a pure extension and not redefine any item in the parent dictionary so that the file would still work if you would just ignore anything not in mmcif_pdbx.