Dataset taken from mlbench. The inputs are 4 independent variables uniformly
distributed over the ranges
$$0 \le x1 \le 100$$
$$40 \pi \le x2 \le 560 \pi$$
$$0 \le x3 \le 1$$
$$1 \le x4 \le 11$$
The outputs are created according to the formula
$$y = (x1^2 + (x2 x3 - (1/(x2 x4)))^2)^{0.5} + e$$
where e is $N(0,sd)$.