asymmetrize the notation for convex combinations

[t6s]

Motivation for this change

The notation a <| p |> b for the convex combination operation has not been showing
its asymmetric nature. This PR changes it to a <* p |> b, making explicit that p is multiplied to a.

I also suggest closing issue #860 by this PR.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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