Gensim topic modeling as a CLAMS app

[keighrim]

This issue to keep track of development effort for a keyword-extraction/summary app based on gensim topic modeling library.

A few notes;

• The overall structure (I/O spec) of this app should be similar to tfidf or bart-summary app.
• We have an old topic modeling app that was never published (https://github.com/JinnyViboonlarp/app-topic-modeling) Probably the old I/O spec and SDK (in-)compatibility make re-using of the old app mostly impossible, but we still might want to look back what's been done back then.