Use this if you are using igraph from R
hrg creates a HRG from an igraph graph. The igraph graph must be
a directed binary tree, with n-1 internal and n leaf
prob argument contains the HRG probability labels
for each vertex; these are ignored for leaf vertices.
The igraph graph to create the HRG from.
A vector of probabilities, one for each vertex, in the order of vertex ids.
hrg returns an