atomium breaks on RCSB generated assembly files

[samirelanduk]

For some reason, the .cif files for specific assemblies that you can download for assemblies don't have a struct_asym table. atomium needs this table because there is no other way to map atom chain IDs to entity types, so when parsing you get a Key Error.

I am not writing a work around for this now because these assemblies can be generated anyway, but it should be fixed at some point.