suggestBandwith: Suggest bandwith for local polynomial regression
Description
The bandwith is suggested coordinate wise to be
$$0.9 \sqrt{5} \min\left(sd(x),\frac{IQR(x)}{1.349}\right) n ^{-\frac{1}{d+4}} (p+1)$$
where p is the polynomial degree used and n is the number of coordinate points.