fixes #1551 (move isMeasurable from simple_functions.v to measure.v)

[affeldt-aist]

Motivation for this change

fixes #1551

fixes #1558

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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• 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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[affeldt-aist]

@t6s the last commit fixes #1518

To summarize the naming fixes:

measurable_funPT  : measurable_fun [set: aT] f
measurable_funP   : measurable_fun D s
measurable_funPTI : measurable Y -> measurable (f @^-1` Y).
measurable_funP1  : measurable D -> measurable (D `&` f @^-1` [set y])

[affeldt-aist]

ping?

[hoheinzollern]

The changes look good. Wouldn't it make more sense to put the renamed lemmas into the "RENAMED" section of the changelog?

Feel free to ignore if you think it's inappropriate.

[affeldt-aist]

The changes look good. Wouldn't it make more sense to put the renamed lemmas into the "RENAMED" section of the changelog?

Feel free to ignore if you think it's inappropriate.

I agree it's clearer.