The NCDF_GROUPDEF function creates a new empty group and assigns it a name.
            The NetCDF 4 format allows you to create groups within data files to better organize the data within the file. After defining a group, you can define variables, attributes, and dimensions within the group.
            Examples
            The following example creates a NCDF 4 file in IDL’s current working directory, and adds parent and child groups to the file.
            filename = FILEPATH('nc_groupdef_file.nc')
id = NCDF_CREATE(filename, /NETCDF4_FORMAT, /CLOBBER)
g1 = NCDF_GROUPDEF(id, 'group1')
g2 = NCDF_GROUPDEF(id, 'group2')
g11 = NCDF_GROUPDEF(g1, 'group1-sub1')
g21 = NCDF_GROUPDEF(g2, 'group2-sub1')
NCDF_CLOSE, id
            Syntax
            Result = NCDF_GROUPDEF( Cdfid, Name )
            Return Value 
            Returns the ID of the newly-created group.
            Arguments
            Cdfid
            The NetCDF ID of the parent group, returned from a previous call to NCDF_OPEN, NCDF_CREATE, or NCDF_GROUPDEF.
            Name
            A scalar string containing the new group name.
            Keywords
            None
            Version History
            
            See Also
            NCDF_DIMIDSINQ, NCDF_FULLGROUPNAME, NCDF_GROUPNAME, NCDF_VARIDSINQ