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. Note that renorm will not work with pileup
since increasing the normalization does not linearly increase the predicted
count rate. Therefore you should set renorm none prior to doing a
fit with pileup.

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 par4 where p is the number of piled photons) |

par5 | PSF fraction. Only this fraction will be treated for pile-up. Note
that this is not the fraction of the PSF included in the extraction region
but is the fraction of counts in the region which are from the point source
whose pile-up is being modeled. For this model to work well the extraction
region should be large enough to contain essentially all the PSF. |

par6 | Number of regions. The counts to be piled-up will be distributed among par6 regions, which will be piled-up independently. |

par7 | Value of FRACEXPO keyword in ARF. |