Incidence mechanism for properties

[RobertJacobsonCDC]

The ValueCountIndex takes care of prevalence, but we don't have a built-in mechanism for incidence. The incidence mechanism would count how many times a property value transitions TO a value. This would accumulate until "reset". The external API would have client code provide a period, and the incidence would be reset according to that period. At the end of a period, the data would (presumably) be reported.

This replaces tabulator.