Started implementation of Giry monad

[alejandroag]

Motivation for this change

Initial PR for an implementation of the Giry monad after discussion with @affeldt-aist and @hoheinzollern (See also PR #1177)

Co-authored with @markusdemedeiros.

This is WIP and not ready to be merged.

Checklist

• [ ] added corresponding entries in CHANGELOG_UNRELEASED.md

• [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• 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.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
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