Implement IsJoin/MeetIrreducible for vertices in lattice digraphs

[wilfwilson]

...and for other kinds of digraphs where the notion makes sense, such as maybe join/meet semilattice digraphs, and possibly even partial order digraphs in general.

And correspondingly, there could be an attribute JoinIrreducibleVertices, etc.

This is just an idea!

There's a very slight possibility that this could ultimately be used by the Semigroups package, where there exist functions such as IsJoinIrreducible and JoinIrreducibleDClasses.

The notion of join/meet primality might also be good to have https://en.wikipedia.org/wiki/Lattice_(order)#Important_lattice-theoretic_notions

[james-d-mitchell]

@ThatOtherAndrew you might want to do take a look at this