Hide solution code in diffusion and advection notebooks

[mdpiper]

It would be good to hide the solution code in the Python diffusion and advection notebooks, similar to what @mcflugen has done in the Landlab notebooks; e.g.:

Click for a solution

# Visualize links
plot_graph(rmg, at="link")

This will give students a chance to reason out solutions on their own, then compare with our suggested solutions.