Better way to have answers to exercises

[KrisThielemans]

@gschramm has the following tip:

We discussed having code block hints that users could unhide if they are stuck, an easy way to do this would be to have another folder in the repository called something like snippets then the code # %load './snippets/01_exA.py' would load the contents of 01_exA.py into the jupyter notebook cell. We usually give instructions like Uncomment the following line and run the cell to see the solution, to run the lines you'll need to run the cell a second time