Fit the Bradley-Terry model using the EM or MM algorithm
BT_EM(W, a, b, maxit = 5000L, epsilon = 0.001)
a K*K square matrix of class "dgCMatrix"
the shape parameter of the gamma prior
the rate parameter of the gamma prior
the maximum number of iterations
controls the convergence criteria
A list containing a K*1 matrix with the pi estimate, the N matrix, the number of iterations, and whether the algorithm converged.