datatree ops.py migration cleanup

[flamingbear]

What is your issue?

During the 3/26/2024 design discussion meeting (#8747), we discussed the monkey patching of methods that was required to wrap datatree nodes with the desired Xarray API function. This was a primarily done as a necessity due to the datatree code not living in the xarray repository. The better implementation could be to add the wrapping to xarray's generate_aggregations.py module. The ultimate decision made by Stephan and Tom was to add this as a future issue (this one) and migrate the file as is for now.