cpflux: calculate photon flux

A convolution model to calculate the photon flux of other model components. For example :

   cpflux*phabs*(pow + gauss)

with the normalization of the power-law model fixed to a non-zero value gives the photon flux and error on the entire model.

   phabs*cpflux*(pow + gauss)

again with the normalization of the power-law fixed to a non-zero value gives the unabsorbed photon flux and error. Finally,

   phabs*(pow + cpflux*gauss)

with the normalizaton of the gaussian fixed to a non-zero value gives the photon flux and error on the gaussian component. Note that when the cpflux model is used the normalization of one of the additive models must be fixed to a non-zero value. It is also important to ensure that the energy range over which the model is calculated (which is determined by the response matrix in use) covers the energy range for which the photon flux is calculated. If the model to which the cpflux is applied integrates to zero then a divide-by-zero error will occur resulting in NaN values for the fit statistic.

Parameters are:

par1=Emin	Minimum energy over which photon flux is calculated.
par2=Emax	Maximum energy over which photon flux is calculated.
par3=Flux	Photon flux in ph/cm$^2$/s