Add function to restrict a real to the intervale [0 , 1]

[lyonel2017]

Motivation for this change

Add function to restrict a real to the interval [0, 1].

Require for #2064

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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[affeldt-aist]

What to you about generalizing to realDomainType and any min/max?
Since 0 <= 1 is registered as a Hint, you could use Local Notation clamp01 := (@clamp R 0 1)
and this should be transparent.

[lyonel2017]

What to you about generalizing to realDomainType and any min/max? Since 0 <= 1 is registered as a Hint, you could use Local Notation clamp01 := (@clamp R 0 1) and this should be transparent.

This works for me.

[affeldt-aist]

This is used in PR #2064 and something to have around for probability theory.
Thanks for the contribution.
Note that ideally this should be PRed to MathComp (fyi: @proux01 @CohenCyril ).