Follow-up from "Implement the splitting of votes"

The following discussions from !79 (merged) should be addressed:

@eugenz started a discussion: (+2 comments)

I've made minor changes to the documentation in this branch: keep the overview high-level, polish the paragraph about ballots. Feel free to use it if you agree. (I thought it is simpler in this way than to add comments.)

By the way, I find "weight" slightly better than "fraction", because the x_fraction only specifies the fraction nominator, the denominator is implicit. But that's minor.
@mbouaziz started a discussion:

We need a uint_parameter, or manually check that each value is non-negative
@mbouaziz started a discussion:

Nit: of_int Constants_repr.fixed.votes_per_roll could have been done once for all before the loop
@mbouaziz started a discussion:

You can move Int32.of_int in allocate

Could you also add tests:

Edited May 18, 2020 by Mehdi Bouaziz