log-likelihood for ols model with AR1 errors
[mlike]=ar1_like(,y,x)
* param = parameter vector (k x 1)
* y = dependent variable vector (n x 1)
* x = explanatory variables matrix (n x m)
* mlike = a scalar equal to -log(likelihood)
f=ar1_like(parm,grocer_y,grocer_x) // example taken from olsar1.