Update mosaic tile generation notebook

Applies some changes to mosaic.ipynb for adjustments I made in the course of
generating static tiles for WikiWatershed/model-my-watershed#3635.
The last section now
- takes a list of zoom levels and loops over them
- uses asyncio.Semaphore, via a decorator, to limit how many tile requests will
  run simultaneously.

I also copied a few variables into the final cell (i.e. COLORMAP and tile_url_template)
so it can be run independently of the of the earlier cells.