dust: dust scattering

A modification of a spectrum due to scattering off dust on the line-of-sight. The model assumes that the scattered flux goes into a uniform disk whose size has a 1/E dependence and whose total flux has a 1/E$^2$ dependence.

par1	scattering fraction at 1 keV
par2	size of halo at 1 keV in units of the detector beamsize