integral_sum and fix integrable_sum

[affeldt-aist]

Motivation for this change

integral_sum was a missing lemma that we noticed when doing the sampling branch, this triggered a generation of integrable_sum, we do not use them in sampling but look nice to have around

Checklist

• [x] added corresponding entries in CHANGELOG_UNRELEASED.md

~~- [ ] added corresponding documentation in the headers~~

Reference: How to document

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[affeldt-aist]

Since this is a generalization and the addition of a natural lemma, we can maybe merge it?