Giry monad for probabilities

[hoheinzollern]

Motivation for this change

Introducing the Giry monad for probabilities, draft PR for now to track progress.

Checklist

• [ ] added corresponding entries in CHANGELOG_UNRELEASED.md

• [ ] added corresponding documentation in the headers

Reference: How to document

Reminder to reviewers

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[CohenCyril]

@hoheinzollern @affeldt-aist shouldn't this work depend on monae now?

[affeldt-aist]

shouldn't this work depend on monae now?

one thing is that monae is not yet ported to mathcomp-analysis 1.0.0, that makes it more difficult @t6s