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recorn: change correction norm for a spectrum

This model is a replacement for and improvement on the old xspec command recornrm. If a correction file is in use for a spectrum then its normalization can be fitted for using this model. The first parameter, which is not variable, is the spectrum number and the second the correction file normalization. The starting value of the second parameter should be set to the current value of the correction file norm (this can be independently set using the cornorm command).

Note that in order to fit the cornorm parameter, the USE_NUMERICAL_DIFFERENTIATION setting in the user's Xspec.init start-up file must be set to true. This causes XSPEC to use a slower full numerical differentiation algorithm when calculating parameter derivatives during a fit, and therefore is not recommended for general usage.

par1 specnum: spectrum number
par2 cornorm: correction file normalization