This function returns the index we want to maximize on the set of potential clusters, for each potential cluster, and each permutation
wmw_uni(rank_data, matrix_clusters)
numeric matrix.
matrix. Matrix of the ranks of the data for all permutations. Each column corresponds to a permutation and each row corresponds to a site or an individual.
numeric matrix. Matrix in which each column represents a potential cluster. It is the result of the "clusters" function.