Returns a list of valid standard_names that are allowed to be unitless

def get_unitless_standard_names():
    '''
    Returns a list of valid standard_names that are allowed to be unitless
    '''
    global _UNITLESS_DB
    if _UNITLESS_DB is None:
        with open(resource_filename('cc_plugin_ncei', 'data/unitless.json'), 'r') as f:
            _UNITLESS_DB = json.load(f)
    return _UNITLESS_DB