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

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[affeldt-aist]

There is certainly more theory to better develop but we'll be coming back to it soon so that is good enough for me now.

[t6s]

I suggest changing the statements of differentiable_?submx and letting users combine it with other functions explicitly by differentiable_comp.