Initialize Protein-->Tensor Functions

[zachcp]

Initial implementation of feature creation from AtomCollection.

trait LMPNNFeatures {
    fn featurize(&self, device: &Device) -> Result<LigandMPNNDataDict>;
    fn to_numeric_backbone_atoms(&self, device: &Device) -> Result<Tensor>; // [residues, N/CA/C/O, xyz]
    fn to_numeric_atom37(&self, device: &Device) -> Result<Tensor>; // [residues, N/CA/C/O....37, xyz]
    fn to_numeric_ligand_atoms(&self, device: &Device) -> Result<(Tensor, Tensor, Tensor)>; // ( positions , elements, mask )
}

impl LMPNNFeatures for AtomCollection {
...
}