fix missing "units" and "positive" metadata attributes in XC_bnds, YC_bnds, and Z_bnds #50
Description
fix missing "units" and "positive" metadata attributes in XC_bnds, YC_bnds, and Z_bnds
in v4r4 files, those extra metadata fields are not in the netcdf files' metadata, despite Z_bnds having 'units' and 'positive'.