Lfun1_integrable turned into an equivalence

[affeldt-aist]

Motivation for this change

This is a strict generalization so I'll maybe merge if CI is green.
This observation comes from the sampling branch.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• 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.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels