DERRF may yield values outside of min max arguments

[Blunde1]

Describe the bug
Running

y=0.5 * (1 + math.erf((x + _skew) / (_width * np.sqrt(2.0))))

for x+_skew>=9 and _width<1 (the relative size is important) yields a result of exactly y=1.0 due to precision.
This means that the largest value will be y=max+1>max after binning (using floor) and division.

Expected behaviour
All returned values should be between min max for both errf and derrf.
Make tests pass

Screenshots