Added differentiability of the max function

[lstrsrmn]

Added a lemma for showing \max is differentiable

Added two lemmas for differentiating the max function when the order is known at the point.

Motivation for this change

Differentiability of max can be useful and is general enough it should be added to the library.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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• 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.

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