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").