fix itv_closed_ends

[t6s]

Motivation for this change

set_interval.itv_closed_ends is wrongly defined. This PR fixes it.

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

• Read this Checklist
• Put a milestone if possible
• Check labels

[affeldt-aist]

It might be good to indicate in the changelog that the definition has changed following a bug fix.

[t6s]

It might be good to indicate in the changelog that the definition has changed following a bug fix.

added