NumeratorSNRCHIPDF: preserve histogram count in .__iadd__()

- the count is meaningless, it is only used to set the scale of the density estimation kernel, but for that reason it should not change as the result of addition operations
- in the current pipeline care is taken to never add more than a single copy of the numerator into any ranking statistic in order to control the total count
- this patch causes .__iadd__() to preserve the total count, allowing the analysis pipeline plumbing to be simplified since there is no reason to fear adding multiple copies of the numerator together