Moved gsl-mfunction, def-mfunction, and gsl-mfunction-fdf from
roots-multi.lisp to generic.lisp because they are common to several
files in the solve-minimize-fit module. Made arguments default
instead of required in #'linear-mfit, linear-mfit-svd,
weighted-linear-mfit, weighted-linear-mfit-svd. Most of these do not
have examples, so untested. This required new functions
#'default-covariance, #'default-lls-workspace. Renamed examples to
#'linear-least-squares-univariate-example and
#'linear-least-squares-multivariate-example, and changed to
'print-details. Revised definitions in nonlinear least squares to
streamline data flow; there is no setup function or global
pre-declaration of the number of parameters or observations required
anymore. The example function now called
nonlinear-least-squares-example will take the number of observations
and the fitter method as optional arguments.