Initial work on refactoring branch pruning logic

[MarkBaker]

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?

Refactoring of the branch pruning to extract the logic from the Calculation Engine. This is to facilitate detailed investigation of Issue #2002 and Issue #2146 (as well as reducing some of the complexity of the calculation engine)

[oleibman]

Wow, you certainly have been busy!

Can you add issue #1971 to this investigation?

[MarkBaker]

@oleibman It isn't the same issue; but I'll add it to the list of CalcEngine oddities that I need to investigate: there's a couple of other's that I'll look at addressing once I've solved this problem with null empty function arguments and branch pruning.