Add more methods

[paskn]

See this review https://dl.acm.org/doi/full/10.1145/3623269

[paskn]

See also Derek Greene's work:

• https://github.com/derekgreene/topic-stability
• https://derekgreene.com/slides/topic_stability_may2014.pdf
• https://github.com/derekgreene/topicscan

[paskn]

Can we compare topics with embeddings? We could (1) merge top-n docs + top-n terms, (2) get embeddings for this text, (3) measure topic-topic similarities on these embeddings. Drawbacks include: may be very slow.