This function is a switcher among various numerical fitting functions
(ols.fit.cg, ols.fit.chol, ols.fit.qr,
ols.fit.svd and ols.fit.sweep). The argument method
does the switching: "qr" for ols.fit.qr, etc. This should usually
not be used directly unless by experienced users.
Usage
ols.fit(x, y, method = "qr", tol = 1e-7, maxiter = 100)