fixes #1313 and moves Egorov

[affeldt-aist]

Motivation for this change

fixes #1313

This PR also shuffles the contents of the file lebesgue_measure.v so that lemmas that do not depend on Lebesgue's measure appear before it (the Egorov theorem in particular).

Checklist

- [ ] added corresponding entries in CHANGELOG_UNRELEASED.md

• [x] added corresponding documentation in the headers

Reference: How to document

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[affeldt-aist]

I also think that lebesgue_measure.v should be split, it not well named anyway, its first part is about real-valued measurable functions, I didn't go as far as appending its first part to measure.v because the latter is already very long, a new file like measurable_realfun.v might be better, what about recording the splitting idea as a issue for later (unless you think it is a better idea to do it now)?

[zstone1]

I think a measurable_realfun.v would be great. But it doesn't have to happen now. An issue is sufficient.