Set the type of a Sequence if it has not been set.

def set_type(self, sequence_type):
        '''Set the type of a Sequence if it has not been set.'''
        if not(self.sequence_type):
            for id, r in self.sequence.iteritems():
                assert(r.residue_type == None)
                r.residue_type = sequence_type
            self.sequence_type = sequence_type