Match error disorder

[multimeric]

This fixes the error that has a message like this:

ERROR//13:26:29.697//OpenBabel OBAtom with serial number 8859 could not be matched to a BioPython counterpart.
Traceback (most recent call last):
  File "/home/migwell/miniconda3/envs/arpeggio/lib/python3.7/site-packages/arpeggio/core/interactions.py", line 1996, in _establish_structure_mappping
    biopython_atom = serial_to_bio[serial]
KeyError: 8859

This happens because the protein_reader module treats ? in the label_alt_id as though the atom has an alternate atom, and therefore is disordered, which ultimately messes up the ID mapping. However I believe that it's much better to treat ? as though it is not disordered, and indeed if I do this, it fixes run errors that I encounter. Notably this is not a duplicate of #4, which has the same exception class, but is related to the fact that PDB input is broken. This error happens with .mmcif files instead.