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step: step function convolved with gaussian

A step function convolved with a gaussian.

$\displaystyle 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