fix: sample_pa() respects the out.seq and out.dist arguments again, regression introduced in igraph 2.0.0

[szhorvat]

Closes #1224.

@krlmlr Let's hope CI passes. Can you please help with adding a test? While this is a stochastic function, sample_pa(3, out.seq=0:2) should always output the same thing as make_full_citation_graph(3)

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[krlmlr]

Thanks for taking this on!

[szhorvat]

Would this work as a test?

identical_graphs(sample_pa(3, out.seq=0:2), make_full_citation_graph(3))

I forgot about identical_graphs(). This is a stronger test than what we already have.

[krlmlr]

We'd want expect_identical_graphs() for this to be idiomatic. In the next iteration.