The cross-validation function with leave-one-out estimator.
bw: array_like :
func: callable function :
L: float :
Calculates the cross-validation least-squares function. This function is minimized by compute_bw to calculate the optimal value of bw.
For details see p.35 in 
where is the leave-one-out estimator of g(X) and is the vector of bandwidths