Add Ability to Add Custom Functions to Calculation

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(and ways that people have coped with its absence), see issue #2900 and issue #4048; also PR #4043 which will be superseded by this PR.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

Author: oleibman

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Added
 
-- Nothing yet.
+- Add ability to add custom functions to Calculation. [PR #4390](https://github.com/PHPOffice/PhpSpreadsheet/pull/4390)
 
 ### Removed
 
@@ -36,7 +36,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 ### Added
 
 - Support Justify Last Line. [Issue #4374](https://github.com/PHPOffice/PhpSpreadsheet/issues/4374) [PR #4373](https://github.com/PHPOffice/PhpSpreadsheet/pull/4373)
-- Allow Spreadsheet clone. [PR #437-](https://github.com/PHPOffice/PhpSpreadsheet/pull/4370)
+- Allow Spreadsheet clone. [PR #4370](https://github.com/PHPOffice/PhpSpreadsheet/pull/4370)
 
 ### Changed