set nuopc gridmaps to unset for online generation

Currently in grids.py we set all gridmaps to 'idmap' and then overwrite that idmap setting for each gridmap match found, we then print a warning if 'idmap' is found but a gridmap is expected.
I propose that if the driver is nuopc instead of printing a warning we change 'idmap' to 'unset' here to allow for internal generation of the mapping file.  