Return the loci that are in clusters that have changed between
        partitions p1 and p2

def get_changed(self, p1, p2):
        """ 
        Return the loci that are in clusters that have changed between
        partitions p1 and p2 
        """
        if p1 is None or p2 is None:
            return list(range(len(self.insts)))
        return set(flatten_list(set(p1) - set(p2)))