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

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

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• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

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[affeldt-aist]

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

[t6s]

good