step: step function convolved with gaussian

A step function convolved with a gaussian.

$$A(E) = {K\over{2}}[1-\mbox{erf}({(E-E_s)\over{\sqrt{2}\sigma}})]$$

par1	$E_s$, start energy (keV)
par2	$\sigma$, gaussian sigma (keV)
norm	$K$, step amplitude