In the documentation give URLs for the RI curves, etc.

[andyfaff]

In getting started there are relative links provided for loading dispersion curves, e.g. '../materials/void.csv'. The user won't know where to get those files from, because it they've pip install refellips they won't be on a user's computer.

• Can you amend getting_started.ipynb to include a URL for the dispersion curves you load? Either from dispersion.info, or the refellips repo
• The same argument goes for the example datafile we analyse