Extract all Error functions from the Engineering class into a dedicated Erf and ErfC classes

[MarkBaker]

Extract all Error functions from the Engineering class into a dedicated Erf and ErfC classes

Retain the original methods in the Engineering class as stubs for BC, but deprecate them. They will be removed for PHPSpreadsheet v2

Note that unit tests still point to the Engineering class stubs; these should be modified to use the Erf and ErfC classes directly when the stubs are removed

This is:

- [ ] a bugfix
- [ ] a new feature
- [ ] code restructuring and 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?

This is a refactoring to reduce complexity of the Excel function classes in PHPSpreadsheet, making them easier to maintain in the future