Issues with episode 13 #171

Thanks a lot to the maintainers for this really nice lesson! Just want to point out some issues in the Episode 13 ("Plotting in Python"), probably arising from the resolution of some rebase/merge conflicts.

For example:

• text appears twice (e.g. the paragraph "The Matplotlib library can be imported using" appears both in the first and second last section);
• the second-last section has two headers ("Fine-tuning figures with Matplotlib" and "Customising our plots with Matplotlib");
• the first exercise has two "Solution" boxes (the first one of which corresponds to a different exercise);
• I believe that the first example & exercise in the second-last section should be removed?