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 $1+\sqrt{N+0.75}$, 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.