Add lemma le0_expectation_cdf

[Yosuke-Ito-345]

Motivation for this change

Add lemma le0_expectation_cdf, which is the counterpart of lemma ge0_expectation_ccdf.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md
  N.B. I created CHANGELOG_UNRELEASED_new.md because the current CHANGELOG_UNRELEASED.md appeared to be old.

• 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

[Yosuke-Ito-345]

@affeldt-aist @t6s
I would appreciate your review. m(. .)m