Created reader for PDBx/mmCIF files

This should be viewed as very preliminary.  The data model is complex and not documented in a way that makes it easy to understand.  This seems to work on a handful of files that I've tested, but it needs a lot more testing.

I'm a little torn about whether to include this in 6.2.  It would be really nice to have it, but it would need a lot of testing during the beta period.  Any thoughts?