Better Definitions for Mixed Parameters and Values Part 1 of Many

[oleibman]

Improve documentation within program by making explicit what types of values are allowed for variables described as "mixed". In order to avoid broken functionality, this is done mainly through doc-blocks. This will get us closer to Phpstan Level 9, but many changes will be needed before we can consider that.

Executable code change is to add 2 new methods to Cell - getValueString and getCalculatedValueString, which are the same as the methods without the String suffix, except that they always return a string result. These are invoked at several places in the code where it makes sense, in particular by Writer code.

This is:

• a bugfix
• improved API documentation
• 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

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.