calculates the Wilks lambda = inter-classes inertia / total inertia
w_lambda = wilks(x,y_classes)
a matrix (n x q)
the class of each observation; a conjunctive vector (n x 1) or a disjunctive matrix (n x nclasses)
Wilks lambda = inertia within classes / total of inertia ; a scalar between 0 and 1
w_lambda=0: classes very well separated; w_lambda=1: classes not separated