gstlal_inspiral: set numerator in ranking stat output

- when collecting trigger histograms from scratch, initialize the numerator with the signal model so that the resulting ranking statistic file is closer to being in a usable state as-is, without having to mix in the output of gstlal_inspiral_create_prior_diststats
- this has been made possible by the recent change to the numerator's .__iadd__() method that causes overall histogram count to be preserved across marginalization operations
- teach gstlal_inspiral_pipe to disable numerator initialization in the create_prior_diststats jobs since it is not longer required