EarthMover DAP2 implementation compatibility

[lesserwhirls]

Versions impacted by the bug

All

What went wrong?

In the EarthMover implementation of the DAP2 das response (example), Not a Number values for attributes are represented by the string nan. However, in Java and other languages, the string representation is NaN, and thus netCDF-Java fails to parse the time variable:

time {
  String long_name "time";
  String axis "T";
  String calendar_type "GREGORIAN";
  String units "days since 1993-01-01";
  String calendar "gregorian";
  Float64 _FillValue nan;
}

Relevant stack trace

[14:35:52.892 ERROR ucar.nc2.ui.ToolsUI] NetcdfDatasets.open cannot open https://compute.earthmover.io/v1/services/dap2/NOAA-PMEL/cefi-nep-hindcast-monthly/main/regrid/main/opendap
java.lang.IllegalArgumentException: Illegal Numeric Value for Attribute Value for _FillValue
	at ucar.nc2.dods.DODSAttribute.<init>(DODSAttribute.java:88)
	at ucar.nc2.dods.DodsV.addAttribute(DodsV.java:443)
	at ucar.nc2.dods.DodsV.addAttributeTable(DodsV.java:433)
	at ucar.nc2.dods.DodsV.parseDAS(DodsV.java:406)
	at ucar.nc2.dods.DODSNetcdfFile.<init>(DODSNetcdfFile.java:241)
	at ucar.nc2.dods.DODSNetcdfFileProvider.open(DODSNetcdfFileProvider.java:24)
	at ucar.nc2.dataset.NetcdfDatasets.openProtocolOrFile(NetcdfDatasets.java:431)
	at ucar.nc2.dataset.NetcdfDatasets.openFile(NetcdfDatasets.java:347)

Relevant log messages

No response

If you have an example file that you can share, please attach it to this issue.

If so, may we include it in our test datasets to help ensure the bug does not return once fixed?
Note: the test datasets are publicly accessible without restriction.

Yes

Code of Conduct

• I agree to follow the UCAR/Unidata Code of Conduct

[lesserwhirls]

It looks like the DAP4 parsing code will also need to be updated to handle nan string values.