set nuopc gridmaps to unset for online generation

[jedwards4b]

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.