Check air pollution workflow

[LouiseABowler]

The air pollution script isn't in the repo at the moment, so let's add that back in and then make a similar set of changes to those in other branches.

• [x] Add main script
• [ ] Make sure the data source is listed
• [ ] Add command line arguments for parameters, with defaults set to those used in the ICML paper
• [ ] Output a list of the parameters used
• [ ] Put data load process into a separate function so it can be reused when generating the figure
• [ ] General tidy up of main script and the figure generating script