graph that is represented a series
A graph as an arrays of indeces Complexity: O(log(V) (V + E))
import mir.series: series; static immutable keys = ["a", "b", "c"]; static immutable data = [ ["b", "c"], ["a"], ["b"], ]; static immutable graphValue = [ [1, 2], // a [0], // b [1], // c ]; assert (series(keys, data).rcgraph == graphValue);