Add an attribute to a variable caring about the type
varaddattrk(values, ncfile, varn)
values = [attrname]|[attrvalue]|[attrk]
attrname = name of the attribute
attrvalue = value of the attribute
attrk = 'S', string ('!' as spaces); 'I', integer; 'R', real; 'D', double;
'npR', numpy Real, 'npR32', numpy Real-32
ncfile = netCDF file
varn = variable name
$ python ${pyHOME}/nc_var.py -o varaddattrk -f ${finaloname} -S 'longitude_of_central_meridian|-60.0|R' -v Mercator