cif2ncmat should support mmcif format

[tkittel]

mmcif files (larger biological structures) are not exactly the same format as CIF files, so out of the box we don't support them currently. However, gemmi can read these files, so it might be possible.

So far I noted that one has to replace the call to .make_small_structure_from_block(..) with .make_structure_from_block(..). We might be able to tell which one to use by first calling the block's .get_mmcif_category_names() method (not sure).

[tkittel]

Attaching example of such a file:

3q3l.cif.gz