iPython notebooks for existing examples

[itrharrison]

It would be good to have iPython notebooks showing the results of the examples to guide the user through.

Something like this (for the MCCL galaxies):
#284

This could also be linked to a binder (a la https://mybinder.org/v2/gh/ntessore/skypy-0.3-demo/master) allowing the user to interact with the notebooks online as well.

This could also be expanded in future to new examples, other demos etc.