Returns the frequency at which a particular GO term has
            been observed in the annotations.

def get_term_freq(self, go_id):
        '''
            Returns the frequency at which a particular GO term has
            been observed in the annotations.
        '''
        num_ns = float(self.get_total_count(self.go2obj[go_id].namespace))
        return float(self.get_count(go_id))/num_ns if num_ns != 0 else 0