slightly enhanced convolution of piecewise functions

[fchapoton]

This is reworking the code for convolution of piecewise functions. Hopefully simpler and a bit faster.

:memo: Checklist

• [x] The title is concise and informative.
• [x] The description explains in detail what this PR is about.
• [ ] I have linked a relevant issue or discussion.
• [x] I have created tests covering the changes.
• [x] I have updated the documentation and checked the documentation preview.

[github-actions[bot]]

Documentation preview for this PR (built with commit b1f4293a8216d45d9271a9906a954eba75091c7a; changes) is ready! :tada: This preview will update shortly after each push to this PR.