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

Merge policy

As a rule of thumb:

• 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
• Check labels