Formatting

[reubenharry]

A standard practice is to use an automatic code formatter when writing code. This PR has run black (one such formatter) on all the Python code. This standardizes the code, which is generally nice, with the downside that it inevitably makes a few formatting decisions that not everyone loves.

We can discuss on Monday whether to use a formatter going forward. If so, I'll automate it, so that it runs when you commit code.

[reubenharry]

The only reason I'm waiting to merge this is that it will be annoying to merge into your ensemble branch (because it will create thousands of tiny inconsequential formatting changes, and some may create conflicts with your branch). For that reason, let's do this once you merge the ensemble code into the main branch.