Converting to PyG dataset

[cshjin]

• [x] #2
• [x] #3
• [x] #4
• [x] heterodata? or data?
• [x] #8
  • pyg.Data
• [x] #9
  • pyg.HeteroData

[cshjin]

Close for now. We model the AC and DC networks as HeteroData in PyG.