Simplify running the dictionary validation tool for external users

[benmwebb]

It would be helpful for external users (such as me) to be able to run the dictionary validation tool on their own structures before submission. This will relieve some of the burden from @brindakv, particularly as the number of structures in PDB-Dev grows.

My understanding is that in order for this to work, the PDBx and IHM mmCIF dictionaries currently have to be manually merged. It would be most helpful if the tool could instead handle this internally, so that updates of the IHM dictionary are more straightforward.