Returns the variable for longitude

    :param netCDF4.Dataset nc: netCDF dataset

def get_lon_variable(nc):
    '''
    Returns the variable for longitude

    :param netCDF4.Dataset nc: netCDF dataset
    '''
    if 'longitude' in nc.variables:
        return 'longitude'
    longitudes = nc.get_variables_by_attributes(standard_name="longitude")
    if longitudes:
        return longitudes[0].name
    return None