Computes the quadratic discriminant of each mixture component,
quad_disc(Y, theta.alpha, theta.mu, theta.sigma)A matrix of size n x p.
The alpha values: An array of K positive real numbers they must verify the condition sum(thetaOld.mu)== 1.
The estimated centers: A list with K elements, each of them is an array of length p.
The estimated scatter matrices: A list with K matrices, each of them has dimension p x p
A n x K matrix, where each row has the values of the quadratic discriminant with regarding to the j-th mixture component, j = 1,...,K