Extend fraction to weighted samples

We have a fraction accumulator now, but it does not work correctly for weighted samples. However, we definitely need this in high-energy physics because we often estimate the efficiency of some detector from weighted simulation samples.

There are several ways how this could be implemented
1) New class `weighted_fraction`
2) Specialization `fraction>`
3) Template specialization `fraction`

Number 3) is my least favored. I like 2), if that's feasible.

Ping @henryiii @gohil-jay @jpivarski