Groupby of expression by dataarray leads to non-xarray like behavior

[FabianHofmann]

Given a dataarray as input to LinearExpression.groupby, there is different behavior in the operation when setting use_fallback to True or False. While True leads to the standard xarray operation, False converts the dataarray to a pandas dataframe whose columns determine the index (multiindex) of the result.

We should align the behavior that when passing a datarray, the xarray format is put out.