Update merge tables to work with biounits

[biomadeira]

We need to update merge_tables to work with biological units as updated here #25

The asym_ids (label and auth) are updated and 2 new columns added to these atom lines with the original auth/label asym ids.
For example chain A and B, becomes A + AA and B + BA

[biomadeira]

@tbrittoborges you are the best to solve this quickly 😉

[biomadeira]

PR #46 sorted this out.

Biological Units can be downloaded by passing bio_unit=True. These mmCIFs have extra columns orig_label_asym_id and orig_auth_asym_id which are used for merging mmCIF Tables with others...