Snap
Given an input, produces an output, which is snapped to key values, when the input is close to it. Used by [DouFloatSliders For example, suppose V is Int, and the values are a percentage. We could snap to 0, 25, 50, 75 and 100. If the threshold were 2, then inputs 23, 24, 25, 26, and 27 would all return 25.