normal_prob cont./reproductive property

[affeldt-aist]

Motivation for this change

This PR provides a proof of the continuity of normal_prob and of the reproductive
property of normal distribution:

This is taken from the PR #1712
and it relies on the field tactic (fyi: @proux01 )

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
• Check labels