Implement IsJoinIrreducible and IsMeetIrreducible

[ThatOtherAndrew]

Adds two new operations to section 10 (Operations for vertices) of oper.gi/oper.gd:

• IsJoinIrreducible(D, v) — returns true if vertex v is not the join of any two distinct smaller nodes in D
• IsMeetIrreducible(D, v) — returns true if vertex v is not the meet of any two distinct larger nodes in D

Both were originally implemented by building the full O(N²) join/meet table, but then optimised to use DigraphReflexiveTransitiveReduction, a function producing a Hasse diagram which allows for a computational shortcut (see SageMath docs below):

https://doc.sagemath.org/html/en/reference/combinat/sage/combinat/posets/hasse_diagram.html#sage.combinat.posets.hasse_diagram.HasseDiagram.find_nontrivial_congruence

Tests are merged into tst/standard/oper.tst, covering N5, a chain, B2, M3, and error handling (two test case sets written by myself and the rest with LLM assistance because I do not think my mathematical knowledge of the underlying concepts is strong enough to come up with reasonably comprehensive/diverse test cases).

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Closes #400

[ThatOtherAndrew]

Requesting a review from @james-d-mitchell please - how does this look? Is the PR description sufficiently detailed and mathematically accurate?