identifier for completed lebesgue measure

[affeldt-aist]

Motivation for this change

This PR introduces an identifier for the completed lebesgue measure and a proof that the completed sigma-algebra is the same as the Caratheodory one.

Checklist

• [x] added corresponding entries in CHANGELOG_UNRELEASED.md

• [x] added corresponding documentation in the headers

Reference: How to document

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