Combine two clusters into a single cluster.

        Args:
            other_cluster (Cluster): The second cluster to combine.

        Returns:
            (Cluster):   The combination of both clusters.

def merge( self, other_cluster ):
        """
        Combine two clusters into a single cluster.

        Args:
            other_cluster (Cluster): The second cluster to combine.

        Returns:
            (Cluster):   The combination of both clusters.
        """
        new_cluster = Cluster( self.sites | other_cluster.sites )
        new_cluster.neighbours = ( self.neighbours | other_cluster.neighbours ).difference( new_cluster.sites )
        return new_cluster