[jatkinson1000]

This contains a number of hotfixes for the BAS workshop.

• Small typos,
• missing images,
• incorrect (old) dataset link,
• code bugs in exercises,
• Add light background to dark images and,
• actions to build the slides.

Would be good to get these in to update slides now.

Notes from workshop:

• Makes it explicit which 4 files are required from the HadCRUT dataset
• Updates the dependencies list to remove numpy as pointed out by a participant that it is not top-level dependency.