added a few lemmas about sup

[holgerthies]

Motivation for this change

Added a few Lemmas about sup to reals.v.
sup_ge0 is a specialization of ub_le_sup exploiting that sup is set to 0 if there is no supremum,
but it might be useful to shorten some proofs.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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[mkerjean]

Great, thank you for this pull request, it's very useful! I think the proofs can still be shortened a bit, I'll suggest changes as soon as possible.