Start splitting some of the basic Statistical functions out into separate classes

[MarkBaker]

Start splitting some of the basic Statistical functions out into separate classes containing just a few similar functions

This is:

- [ ] a bugfix
- [ ] a new feature
- [X] refactoring

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Part of the refactoring work to move all Excel function methods out of the monolithic category classes and into individual classes, or small function group classes for ease of maintennce