Update notebooks file to work with latest master

[T0T4R4]

I can't find a single one of the notebook files in this repo that work with the latest master.

[timothydmorton]

Yes, many are old/outdated; I haven't (ever?) cleaned the notebooks directory out. https://github.com/timothydmorton/isochrones/blob/master/notebooks/sun-compare.ipynb should work with current master if you want a demo. Also, the rtd-hosted docs should be more or less up to date.