intransparent error message detected

When running RGDR.fit() with one of the inputs being a xr.Dataset instead of an xr.DataArray, it returns the following error message:
```AssertionError: Field and target dims do not match```

We could check for dtype instead and return an appropriate error.
I think this is an easy mistake to make, as the resample returns a dataset even though you insert an xr.DataArray (any reason to do so?). 