Added basic helix parsing for mmCIF

[David-OConnor]

Some notes:

• No sheets yet (There are multiple sheet tables, and I'm not sure how to handle at this time)
• I'm not sure how to correctly assign the start and end IDs based on chain. Maybe something to do with "label" vs "auth" ids? Or maybe the intent is to infer from the text fields. (mmCIF format issue; not code issue). The auth ids line up with the labels ("serial number") used elsewhere, but those are chain-specific... hence my concern.
• Lots of repetition with the atom table parsing due to the way the macros are set up inside fns. I wasn't sure how to properly exfiltrate them to re-use.
• No .PDB support
• I didn't include AA information or length, but IMO it's redundant with the indices. (An overconstraint)