element parsing and channels

[aangaji]

1. Element parsing: changed encoding to s3 (for instance Ca, Cl, but also weird ones like Uut)
2. Element channels:
   - there are three pairs, i tried to update all holograms/holograms_zach/zernikegrams
   - element channels declared in get_ file and passed as kwrg to get_hologram method
   - new Unk channel for elements other than those that have a dedicated channel

• feel free to change element channels, P, F, and Cl just seem to be the most common ones
• there might still be some hard-coded shapes from the old channels which don't work with arbitrary number of channel
• pls make sure this branch works before merging (or just don't merge and train a new model on this branch)