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_CHAIN_RULE setting in the user's Xspec.init start-up file
must be set to `false`. 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 |