Some fixes for Specht modules and diagrams

[tscrim]

📚 Description

This fixes a bug in the Specht modules when the cells of the diagram are bigger than the number of boxes:

sage: SGA = SymmetricGroupAlgebra(QQ, 2)
sage: SGA.specht_module([(2,2), (5,5)])
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
...
IndexError: list index out of range

We also fix a few typos in combinat/diagram.py.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[tscrim]

@darijgr This is the fix to Specht modules (and to diagram.py) you told me about. The problem was a bit different than you were thinking. The n is correct; it is the number of boxes. However, how I was building/checking the diagram was with the wrong size. In fact, this change can make it much more efficient for "dense" diagrams.

@mkoeppe Can you promote @darijgr to the "triage" team?

[mkoeppe]

Done

[tscrim]

Thank you.

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: f0494de

[darijgr]

These changes are good.

[tscrim]

Thank you.