Set the max frequency for the variant

            If max_freq use this, otherwise go through all frequencies and
            set the highest as self['max_freq']

            Args:
                max_freq (float): The max frequency

def set_max_freq(self, max_freq=None):
        """Set the max frequency for the variant

            If max_freq use this, otherwise go through all frequencies and
            set the highest as self['max_freq']

            Args:
                max_freq (float): The max frequency
        """
        if max_freq:
            self['max_freq'] = max_freq
        else:
            for frequency in self['frequencies']:
                if self['max_freq']:
                    if frequency['value'] > self['max_freq']:
                        self['max_freq'] = frequency['value']
                else:
                    self['max_freq'] = frequency['value']
        return