Added small example for Nemo ref and cfg namelists

in_file_ref = "namelist_ref"

in_file_cfg = "namelist_cfg"

with open(in_file_ref) as f:

    ref_nmls = namelist.parse_namelist_file(f)

with open(in_file_cfg) as f:

    cfg_nmls = namelist.parse_namelist_file(f)

nmls = ref_nmls.copy()

for k in cfg_nmls:

    if k in nmls:

        nmls[k].update(cfg_nmls[k])

    else:

        nmls[k] = cfg_nmls[k]

print(json.dumps(nmls, indent=4))