Table Destructor

[oleibman]

There is a circular reference between Worksheet and Table, which can lead to a memory leak. I tried to break this connection in the Worksheet destructor earlier, but this seemed to cause some sort of error with Phpunit 10 on Github, an error that I am unable to duplicate in my local environments. This PR will allow me to explore the problem to see if I can come up with any useful diagnostic data. If I get to a combination that works, I will consider merging it.

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

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.

[oleibman]

This seems to have worked, so I am moving it out of draft status. Differences between this and prior attempt:

• Don't know if this is a different version of Phpunit 10.
• Added destructor to Table. I can't see why this would matter, but other descendants of Worksheet with potential circular references (e.g. Row and Column) do so.
• Changed Worksheet destructor to empty Table collection directly rather than calling a subroutine which returns self. I can sort of convince myself that this is the change that mattered if I don't think about it too much.