Sidechainnet

* Support custom, user-specified datasets.
* Add original sequence information (3-letter AA codes before AAs are "standardized").
* Make it much easier to reproduce SidechainNet (see scn.create, scn.generate_all) by preprocessing and storing ProteinNet online for user access.
* Change `batch.ress` to `batch.resolutions`
* Adds `pandas` requirement to handle ProteinNet dataset split info.
* Smaller additions and improvements.

See PR 32 for more details.

This release makes sidechainnet pip installable (to be tagged/uploaded immediately following this release post). It also allows users to create SidechainNet data using `scn.create`.

See 16 for more information. Small bug fix from v0.3.1 that impacted self-normalizing NeRF function.

Added functionality to `scn.load` and improved API. Running `create.py` now requires ProDy v2.0.

See 16 for more information.

Fix an issue (15) with StructureBuilders when some angles are 0. Even though this is non-physical, StructureBuilders should still try to produce the structure.

This version makes resolution information accessible through batching. Resolution information was previously accessible through SidechainNet's Python dictionaries and via `scn.load(...filter_by_resolution)`.

Resolution information for proteins in a given batch is accessible via the `ress` key when using DataLoaders to batch data, i.e. `batch.ress == (1.3,2.5,None,3.2)`. `None` represents structures that had no available X-ray resolution information.