Add yarn compute_db

[ribose-jeffreylau]

I don't have commit rights so I couldn't push to the original feature branch.

[ronaldtse]

Thank you @ribose-jeffreylau !

As a follow up to this we will need to delete public/db.json, and then make "yarn build" and "yarn start" run the compute db task as prerequisite.

Hopefully we can also generalize the compute_db task take an input dir and an output path, then we can easily incorporate this task into the Geolexica build.