Sitemap Tier Chunk

One slice of one tier of one market's products (gcg/services/sitemap_tiers.py).

Declared before sitemap_chunk, whose {code} would otherwise take "ie-t1" for a market and answer 404. A tier not offered now has no file: the index does not name it, and the pages in it are not offered.

Cut from the list built once for every market and part (_products), so a file is written from memory: the query behind it is the same for all forty markets, and it is only the prefix that differs.