add Rintegral_gt0 and Rintegral_gt0_itvcc

[t6s]

Motivation for this change

Context: https://rocq-prover.zulipchat.com/#narrow/channel/237666-math-comp-analysis/topic/natmul_inj.2C.20Rintegral_gt_0

This PR adds a lemma stating that the integral of a non-negative, continuous function that is nonzero at least at one point is positive.

Two variants are proved, one for open sets, another for closed intervals.

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.

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