Replace linear model with LGBM, add census 2011 data

[MarcoGorelli]

Here's our notebook in its origin, unedited form

[john-sandall]

Going to merge this in anyway as the code for bringing in the census data and applying the LightGBM model will be very useful if/when I'm next building a GE turnout model (may not be so far away!).

[john-sandall]

OK I've now added this here: https://github.com/john-sandall/democracy-hackathon/blob/master/turnout_model/LGBM%20%2B%20census%20data.ipynb

Let me know if you're uncomfortable with this being up. I saw from your Slack comment there may be some evaluation issues, but it will be extremely useful to refer to this in a few months time if a snap election is called!