Join groups of the same element type.

def join_conn_groups( conns, descs, mat_ids, concat = False ):
    """Join groups of the same element type."""

    el = dict_from_keys_init( descs, list )
    for ig, desc in enumerate( descs ):
        el[desc].append( ig )
    groups = [ii for ii in el.values() if ii]
##     print el, groups

    descs_out, conns_out, mat_ids_out = [], [], []
    for group in groups:
        n_ep = conns[group[0]].shape[1]

        conn = nm.zeros( (0, n_ep), nm.int32 )
        mat_id = nm.zeros( (0,), nm.int32 )
        for ig in group:
            conn = nm.concatenate( (conn, conns[ig]) )
            mat_id = nm.concatenate( (mat_id, mat_ids[ig]) )

        if concat:
            conn = nm.concatenate( (conn, mat_id[:,nm.newaxis]), 1 )
        else:
            mat_ids_out.append( mat_id )
        conns_out.append( conn )
        descs_out.append( descs[group[0]] )

    if concat:
        return conns_out, descs_out
    else:
        return conns_out, descs_out, mat_ids_out