Addition of Kähler algebras as a category

[25shriya]

This PR adds Kähler algebras as a category KahlerAlgebras as defined in Theorem 2.28 of this paper. It changes the category of Chow rings to KahlerAlgebras() & CommutativeRings().Quotients(). A method, lefschetz_element() is also implemented for Chow rings which returns a Lefschetz element as provided in Corollary 2.30 of the paper above, which is used for the Hodge-Riemann relations computation.

@tscrim

📝 Checklist

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⌛ Dependencies

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[tscrim]

LGTM.

[tscrim]

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[tscrim]

One new (annoying) thing I think needs to be done is adding new files to the meson.build list file in the categories folder. Some modularity or porting thing that we have to explicitly maintain with extra work for something like this. I don't really understand it, but I think we now have to do it. :/