Sidechainnet

Add fix for create_custom.

Fixes bug introduced in 40 when loading custom datasets.

Quick patch (erroneous print statement) from v0.7.1.

Address an issue with entry [1GJJ_1_A](https://www.rcsb.org/structure/1GJJ) and mentioned in #38. This is a temporary patch, a new version of the data will be released in the future. Thank you for pointing out this issue, wwang2!

1. Implements SCNDataset for easier data access (`data = scn.load(... scn_dataset=True)`).
2. Implements SCNProtein for easier data manipulation (access attributes, etc.).
3. Implements HydrogenBuilder. To add hydrogens to any coordinate set for a protein, simply use `SCNProtein.add_hydrogens()`. Note: the coordinate representation for the all-atom structure with hydrogens (`SCNProtein.hcoords`) does not include terminal-specific atoms (H2, H3, OXT). These are included in generated visualization/PDB file, but not in the coordinate set itself.

Please see the respective functions for more details.

Add protein sequences as strings to Dataloaders via `batch.str_seqs`.