Add IsParadoxical and IsKParadoxical for tournaments

[William-Kenyon]

Fixes #942, for checking if a tournament is paradoxical (every vertex has an in-neighbour), or more generally if it is k-paradoxical (every set of k vertices has a common in-neighbor).

Notes:

• The algorithm for IsKParadoxical(D, k) is brute-force: it checks every subset of size k for a common in-neighbour using list intersection, which has complexity O((n choose k) * k * n) where n is the number of vertices of D.
• I went with the convention that if D has no vertices, then regardless of k, it is not paradoxical, though I don't have any reason for/against the convention.
• Alternatively, if D has vertices but k is 0, then it is considered paradoxical, to not break the pattern that being k-paradoxical implies being (k-1)-, (k-2)-,...-paradoxical.

I made IsParadoxical(D) a property and IsKParadoxical(D, k) an operation, but am wondering if both should be operations, IsParadoxical should be removed, or is there a better option?

A possible follow-up is to add Paley digraphs, which give a way to construct k-paradoxical tournaments (Graham, R. H., Spencer, J. H., "A Constructive Solution to a Tournament Problem").

[james-d-mitchell]

@William-Kenyon can you please update so that the CI passes?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (8d49e9a) to head (7a99180).

Additional details and impacted files

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##             main     #947   +/-   ##
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  Coverage   97.45%   97.45%           
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  Files          50       50           
  Lines       21189    21237   +48     
  Branches      639      639           
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+ Hits        20649    20697   +48     
  Misses        475      475           
  Partials       65       65           

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