Cluster breakends by specified attributes.

def _cluster_by(end_iter, attr1, attr2, cluster_distance):
    """Cluster breakends by specified attributes.
    """
    ClusterInfo = namedtuple("ClusterInfo", ["chroms", "clusters", "lookup"])
    chr_clusters = {}
    chroms = []
    brends_by_id = {}
    for brend in end_iter:
        if not chr_clusters.has_key(brend.chrom1):
            chroms.append(brend.chrom1)
            chr_clusters[brend.chrom1] = ClusterTree(cluster_distance, 1)
        brends_by_id[int(brend.name)] = brend
        chr_clusters[brend.chrom1].insert(getattr(brend, attr1),
                                          getattr(brend, attr2),
                                          int(brend.name))
    return ClusterInfo(chroms, chr_clusters, brends_by_id)