X = NNLS(A,b) returns the vector X that solves A*x = b
in a least squares sense, subject to x >= 0.
A default tolerance of TOL = MAX (SIZE (A)) * NORM (A,1) * EPS
is used for deciding when elements of X are less than zero.
This can be overridden with X = NNLS(A,b,TOL).
[X,W] = NNLS(A,b) also returns dual vector W where
w(i) < 0 where x(i) = 0 and w(i) = 0 where x(i) > 0.
See also LSCOV73GS9WQ, SLASH .