Add MMTF support

[benmwebb]

For smaller output files, it may be useful to encode at least a subset of the IHM dictionary such that it can be written out/read in in MMTF format.

[benmwebb]

This appears to be difficult since MMTF encodes slightly different information than mmCIF itself (so MMTF would need to be extended to include our IHM-specific categories). But the library does now support BinaryCIF, which is a more compact binary encoding of the same information that is stored in mmCIF files.

[aozalevsky]

I believe this one could be closed as "wontfix" since MMTF is officially deprecated.

[benmwebb]

Agreed, we support BinaryCIF instead.