Update mosaic tile generation notebook

[KlaasH]

Proposed Changes:

Applies some changes to mosaic.ipynb for adjustments I made in the course of generating static tiles for WikiWatershed/model-my-watershed#3635.
Most notably, 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 earlier cells.

Related Issue(s):

• AWS 2-6: Generate, Store Static Tiles for Lower Zoom Levels #9

Notes:

• Zoomlevels 0-3 can't be generated with the Lambda tiler because individual tiles take longer than 15 minutes. For those, I used a modified version of the notebook code pointed at a local titiler-mosaicjson server (and limited to concurrency 1. see my comment on #9 for notes on running titiler-mosaicjson locally).
• The tile downloading cell mostly works if you pull it into a script to run it from the command line, but the

  results = await asyncio.get_event_loop().create_task(download_tiles())
  

  line needs to be changed to

  result = asyncio.run(download_tiles())
  

• I'm not sure this is the strategy to pursue long-term. Given that the low-zoomlevel tiles can't be generated with Lambda, it's never going to be possible to wrap this up into a simple script that can just be run against the existing tiler endpoint. And if it's going to require local processing and more-extensive supervision anyway, it might make sense to just switch to a "download the files and make a tile pyramid with GDAL tools" strategy. But that's a question for issue AWS 2-6: Evaluate Adding Hybrid Tiling Endpoint #10.

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.