Add Ability to Add Custom Functions to Calculation

[oleibman]

For an overview of why this is desired (and ways that people have coped with its absence), see issue #2900 and issue #4048; also PR #4043 which will be superseded by this PR.

The list of Excel functions is moved from Calculation/Calculation to its own member. I believe that it is done in a way that will not cause big complications to two experiments from @MarkBaker (PR #2714 and PR #2734). I believe it is also done in such a way that further refactoring of Calculation can follow this model.

Custom functions can be added or removed from the function list. You cannot add a function if it already exists in the list, and you cannot remove a non-custom function from the list. They will, of course, not be understood by Excel if written to a spreadsheet; the use case is mainly using the Calculation engine outside of spreadsheet context.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• 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 and a link to the pull request if applicable
• Documentation is updated as necessary

[oleibman]

4 new false positives from Scrutinizer, which doesn't seem to understand how "protected" works.