pegpwrlw: power law, pegged normalization

A power law with pegged normalization.

$$A(E) = K E^{-\alpha}$$

where :

par1	$\alpha$, photon index of power law (dimensionless)
par2	lower peg energy range (keV)
par3	upper peg energy range (keV)
norm	flux (in units of $10^{-12}$ ergs/cm$^2$/s over the energy par2-par3). If par2 = par3, it is the flux in micro-Jy at par2.