Splatalogue: generate species.json on the fly

See note in https://github.com/astropy/astroquery/pull/754. Since this file has now changed at least once, it makes more sense for it to be produced via query by the user rather than served here.

The issue identifies species.json and points to the note in PR #754; read that note first, then trace where species.json is currently served. Confirm the available query entry point, with done meaning users generate the file through a query rather than receiving a bundled copy.

Written by the indexing model from the issue text.