default fillvalues are not automatically parsed by xarray

[veenstrajelmer]

• is on their radar: default fill_value not masked when read from file pydata/xarray#7723 and Suggestion: Add option for default_fillvals to open_dataset pydata/xarray#2374 and handle default fill value pydata/xarray#2742
• default fillvalue not parsed by xarray, results in negative values in connectivity xugrid#125
• temporary parser created in add default _FillValue attrs to help xarray #495, can be removed when xarray fixes it (and dropping py3.8 support since requiring a new xarray version will not be available for py38)
• currently used in open_partitioned_dataset and layer reconstruction