calculates the adjusted Rand index for two classifications
ri = arand_index(c1,c2);
a conjunctive vector describing a classification
if k classes have been identified, the elements of c1 are chosen among the k first integers
a conjunctive vector describing a classification
if k classes have been identified in c1, the elements of c2 are also chosen among the k first integers
the adjusted Rand index; a scalar between 0 and 1