pileup: CCD pile-up model for Chandra

CCD pile-up model used for brightish point sources observed by Chandra. This is an implementation of the fast pile-up algorithm proposed by John Davis (see http://space.mit.edu/~davis/papers/pileup2001.pdf). The frame time and maximum number of photons to pile up should be fixed. The grade morphing is expressed through a single parameter, alpha, which should be left as a free parameter. This model should be considered in beta test. Note that to calculate fluxes etc. for the model you must remove the pileup component. The pile-up model is similar to the operation of the convolution models, differing only in the treatment of the detector efficiency during the convolution.

par1	frame time (in seconds)
par2	maximum number of photons to pile up
par3	grade correction for single photon detection
par4	grade morphing parameter (good grade fraction is assumed proportional to par4p-1 where p is the number of piled photons)
par5	PSF fraction. Only this fraction will be treated for pile-up
par6	Number of regions. The counts to be piled-up will be distributed among par6 regions, which will be piled-up independently.