[ENH] Clear Notebooks

[ljwolf]

Clear the notebooks so that the user has to run the code in order to see the result on their machine.

This increases the likelihood the user will actually run the code and interact with the work.

If we are concerned, we can also publish all notebooks in html formats to our readthedocs page.

[ljwolf]

This will include incoporating geopandas/scipy2018-geospatial-data#14