multifit residuals

[Patrick Alken]

Hello,

  The function gsl_multifit_linear_svd computes the fit residuals
in order to compute chi^2, but I was thinking it might be useful
to have a separate function to compute the residuals vector since
its often useful for the user to look at all the residuals as a measure
of quality of fit. So basically just reproduce lines 158-166 of
multilinear.c in a new function and store the residuals in a user
supplied vector. Anyone disagree?

Patrick Alken