Reformat demand per bus into an xarray

[trevorb1]

Feature Request

PR #288 introduces sectorally explicit demand files (ie. res-elec, res-heat, ect...). This is outputted as separate csv files, due to me just being more comfortable with pandas than xarray to get the workflow running. This should get switched to xarray though.

Suggested Solution

I believe the dataset should have the coordinates: snapshot, bus, sector, end-use, and the variable will be the demand

Additional Info

No response