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weight: change weighting used in computing statistic

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

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

Standard weighting uses sqrt(N) or the statistical error given in the input spectrum. Gehrels weighting uses 1+sqrt(N+.75), 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.