Discrete and continuous abel transforms

[Tragicus]

Motivation for this change

Abel transforms and a few helper lemmas, as part of #1720.

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.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
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