Extend fraction to weighted samples

[HDembinski]

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<T>
2. Specialization fraction<weighted_sum<T>>
3. Template specialization fraction<T, bool weight_support = False>

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

Ping @henryiii @gohil-jay @jpivarski

[HDembinski]

For those interest, the math for weighted fractions is discussed here https://arxiv.org/abs/2110.00294