Fix rastermap tutorial notebook

[seankmartin]

Hi, thank you for the very nice repository!

I was trying to run the demo notebook (run_rastermap.ipynb) from the Github source code. I ran into some issues, and have posted the fixes that worked for me. It would seem the backend code of rastermap changed since this demo was mode (as there was a reference to an n_Y and model.isort2 that don't seem to exist anymore). I've tried to keep to the original ideas that the notebook was trying to accomplish with the new backend, but it is very possible I have misunderstood the original intention in places.

Anyway, happy to discuss more or make any further changes to this PR - please let me know what you think!