Support for mmCIF Files

[a-r-j]

Is your feature request related to a problem? Please describe.
Currently, we only support PDB files as inputs for protein structure graphs. Large complexes are now unavailable as PDBs.:

Several type of PDB entries are not offered in the legacy PDB format anymore:
Entries containing multiple character chain ids
Entries containing > 62 chains
Entries containing > 99999 ATOM coordinates
Entries that have complex beta sheet topology, see more details

Describe the solution you'd like
Once BioPandas has support for parsing mmCIF files (BioPandas/biopandas#94) , we can parse the DFs into a format consistent with PDB files. This is the simplest route forward. However, mmCIF files are 'better' (esp wrt how they handle insertions / altlocs) as well as author inconsistencies in the contents. Longer term we may consider refactoring to treat mmCIF as the first class citizen input file format.

[mrauha]

The MMCIF -> PDB conversion should be available quite soon in Biopandas, waiting to be merged: BioPandas/biopandas#107

After this is done, using MMCIF's should take couple lines work :)