ugrid output improvements

[veenstrajelmer]

We use xugrid for reading ugrid-compliant data. However, I cannot read shyfem output data since there are some essential attributes missing. I have created a preprocess function in order to let it work automatically: Deltares/dfm_tools#745

Would it be possible to add the necessary attributes to the shyfem output files?

@georgu I see you are the main contributor, could you consider this?