Add an attribute to a variable. Removes previous attribute if it exists
varaddattr(values, ncfile, varn)
values = [attrname]|[attrvalue('!' as spaces)]
ncfile = netCDF file name
varn = name of the variable
$ python ${pyHOME}/nc_var.py -o varaddattr -f SouthAm_Climzones.nc -S 'grid_mapping_name|Mercator' -v Mercator