split probability.v

[affeldt-aist]

Motivation for this change

One step in addressing issue #1808 .

Maybe the file random_variable.v can be further split, or maybe this is not urgent @Yosuke-Ito-345 @hoheinzollern

Changelog not yet done.

The documentation can certainly be improved.

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

[Yosuke-Ito-345]

At this point, I don't think we should split random_variable.v further, but this is not a strong opinion.
By the way, I am planning to move pmf_measurable before the section tail_expectation_formula, since I will use pmf_measurable to prove le0_expectation_cdf (#1808).

[affeldt-aist]

Then unless somebody objects I will prepare the changelog ASAP (by tomorrow?) and merge so that you work more comfortably.