Documentation for Bitset$sample is misleading

[giovannic]

First picked up on here

A rate can mean a lot of things, but in epi. it's most often interpreted as the number of events per unit time. Instead, a probability refers to the likelihood of an event happening to an individual.

At the very least we should document Bitset$sample to have a prob argument instead of a rate argument. But we should change the implementation variable names to make that less confusing for development.