PDBx/mmCIF structure support

[RubenChM]

Current structure file use legacy pdb format which comes with some shortcomings like:

• Number of atoms limited to 999999
• Number of residues limited to 9999
• Number of chains limited to 52
• Name of residues to 4 letters
  • gro files can have bigger residue names like in A01J4

Useful links:

1. Understand:
   1. 101.
   2. Guide.
   3. Ecosystem.
2. Software.
   1. https://gemmi.readthedocs.io/en/latest/cif.html#reading-and-writing