Alter JSON format and make it Linked Data

The original file was valid JSON but it had the library indexed by numbers rather than as an array. That makes it hard to, say, loop over the library unless you assume every top level key is a library item. To solve that, I've converted it into an array of objects under the key `ASCL`.

I've changed the `ascl_id` into the URL rather than just the number code as that is more "webby" - it is now a link to a thing rather than an abstract code.

I've added an attempt at JSON-LD `@context` so that the file has semantic information included.
I'm not sure if `http://dbpedia.org/page/Bibcode` for the bibcode and `http://dbpedia.org/page/Abstract` for the abstract are the best to use but I thought it best to have something.