move example images to the "data" folder

[tobyhodges]

at the moment, the example image files are stored alongside the example scripts in the code folder. They should be moved to data, because that is where people first think to look for them.

[tobyhodges]

Since we decided in #130 to try to write the lesson to be taught in Jupyter, it is appropriate to store all example images in the data directory.

I will move the data files and update the code blocks in the episode files to reflect the change.