missing step argument

add(…).min().max().step() does not work for me. For testing purpose use
a starting value of 0. add() will return the NumberControllerSlider and
the following function calls min() max() step() won’t reach the
corresponding NumberControllerBox.

For the shorthand call add(…, min, max, step) was the step argument
missing in the factory.js. This merge request will fix this but the
other bug remains.