fix: revisit tests for DO operations

[yucongalicechen]

          is the condition "same length xarray" or "on the same `q/tth/d-array`"?  I think it should be the latter or we are adding things incorrectly.  Please can you check the code.  If the code is doing it right, change the doc.  If hte doc is right, we probably should revisit hte tests and code.

Originally posted by @sbillinge in #297 (comment)

[bobleesj]

Could you check the _check_operation? Thats how the code checks for the shape modified by @sbillinge