Model fitting by least squares

Two sections omitted here may be found in SEP-61 p 375. They are ``Multivariate least squares'' and ``Conjugate-gradient descent''. We will build here on the subroutine cgstep() defined there on page 384. (The order of the arguments was revised. It now is cgstep( iter, n, x, g, s, m, res, gg, ss) ).