add add_entry and anti_restrict methods to SkewTableau

[jmcdonough98]

add_entry and anti_restrict are already existing methods in Tableau, but they also make sense for SkewTableau. This PR ports these methods to SkewTableau, and adds a simple check to add_entry to avoid trying to construct a new tableau when the given cell is too large.

:memo: Checklist

• [x] The title is concise and informative.
• [x] The description explains in detail what this PR is about.
• [ ] I have linked a relevant issue or discussion.
• [x] I have created tests covering the changes.
• [x] I have updated the documentation and checked the documentation preview.

:hourglass: Dependencies

None

[trevorkarn]

@jmcdonough98 as long as your tests pass locally, you can set to positive review

[jmcdonough98]

@trevorkarn all tests pass on my machine, but github is not letting me change PR labels. I think I don't have permission.

[vbraun]

merge conflict, please fix

[trevorkarn]

Thanks @vbraun. @jmcdonough98 Do you need help with this?