Vector splitting doesn't support xr.Datasets

[hjabbot]

In playing around with the Duacs dataloader, it appears that the dmag calculations don't work anymore with xr.Datasets. As a hotfix, I have cast the Duacs dataset to be a pd.Dataframe dataloader, but this should be fixed to work with xarray for future dataloader compatibility. Failing that, the docs should mention that vector dataloaders only support pd.Dataframes if vector splitting is required.