isplay   boolean, if true messages will be printed in console.
% opts.lambda    lagrangian multiplier for L1 regularization
% opts.gpu_id    ID of GPU to be used if GPU support is available.
% opts.use_std   calculate least standard deviation instead of L2-norm
% opts.sample    Read about convergence check below!
% opts.tol       -
% opts.tol_      -
%
% Output
% X              Matrix of approximations of the solutions to the
%                   nnls problems. Each row is one solution.
% G              Gram