differentiable rsubmx/lsubmx lemmas + derive_sqrt

[yosakaon]

Motivation for this change

When deriving differential equations using lsubmx/rsubmx, we need proofs of their differentiability.
This PR also contains a lemma making use of the "is_derive1_sqrt" lemma to simply derive square roots.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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