Change the weighting function used in the calculation of chi-squared.

**Syntax: ****weight**
[standard | gehrels |
churazov | model ]

*Standard weighting *uses or the statistical
error given in the input spectrum. *Gehrels weighting* uses , a better
approximation when *N* is small (Gehrels, N. 1986, ApJ 303, 336). *Churazov
weighting* uses the suggestion of Churazov et al. (1996, ApJ 471, 673) to
estimate the weight for a given channel by averaging the counts in surrounding
channels. *Model weighting* uses the value of the model, not the data, to
estimate the weight.