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cutoffpl: power law, high energy exponential cutoff

A power law with high energy exponential rolloff.

A(E) = K E^{-\alpha} \exp(-E/\beta)

par1 $\alpha$, power law photon index
par2 $\beta$, e-folding energy of exponential rolloff (in keV)
norm K in photons/keV/cm$^2$/s at 1 keV

If POW_EMIN and POW_EMAX have been defined by the xset command then the norm becomes the flux in units of $10^{-12}$ ergs/cm$^2$/s over the energy range (POW_EMIN, POW_EMAX) keV unless POW_EMIN = POW_EMAX in which case the norm becomes the flux density in micro-Jansky at POW_EMIN keV. In these cases it is important that POW_EMIN and POW_EMAX lie within the energy range on which the model is being evaluated.