Rintegral atan expR

[affeldt-aist]

Motivation for this change

A few lemmas about Rintegral, atan, and expR that have found their use in an on-going development about the Gauss integral.

Checklist

• [x] added corresponding entries in CHANGELOG_UNRELEASED.md

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

Reference: How to document

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