'Returns volume_stats updated as needed.'

def _append_filter_goodness_functions(self, volume_stats):

    if ('filter_function' not in volume_stats):
        volume_stats['filter_function'] = self.driver.get_filter_function()
    if ('goodness_function' not in volume_stats):
        volume_stats['goodness_function'] = self.driver.get_goodness_function()
    return volume_stats
