Add the option to use bankchisq by adding chisq and bankchisq residuals

RankingStat objects have a new attribute use_bankchisq, which is by default set to False. gstlal_inspiral_create_prior_diststats sets it to True if use_bankchisq: True is provided in the rank section of config.yml. When set to True, the numerator snr_bankchi PDF is used in the numerator's __call__ method, and the denominator ifo_snr_bankchi PDF is used in the denominator's __call__ method. In addition, RankingStat's kwargs_from_triggers returns a dict with chi2s_over_snr2s being this new added-residuals-chisq instead of autochisq.

In order to make this possible, itacac and snglinspiral.c have been modified to store the chisq_norm in the chisq_dof column, and similarly for bankchisq. Then, the bankchisq variable is populated as (chisq * chisq_norm + bankchisq * bankchisq_norm) / (chisq_norm + bankchisq_norm).

Adding the residuals this way has been shown to slightly increase VT when plotted against FAR However, the VT improvement when plotted against SNR is a bit less encouraging, which is why this feature is added as an option

This plot shows that these changes do not change the value stored in the chisq variable:

This plot shows chisq and combined chisq values of triggers:

This plot shows chisq and bankchisq values of triggers: