Pass information about current cluster to aggregation functions

[lazappi]

The node aes aggregation functions currently have no information about the cluster they are working on. This makes it impossible to perform some useful aggregations such as proportions (raised in #54).

One possible solution is to check if the aggregation function has a ... argument. If yes this could be used to pass arguments (or a list) with details of the cluster. Something like:

if ("..." in names(as.list(args(col_aggr_fun)))) {
    cluster_info = list(res = res, cluster = cluster, size = size)
    node_data[aggr_col_name] <- col_aggr_fun(clust_meta, CLUSTER = cluster_info)
}

[lazappi]

This approach doesn't work because you can't be sure what the aggregation function will do with .... For example max() uses it to accept all values to find the max of which causes problems when it gets an extra list.

Still think this would be useful but not sure how to do it.