Clean up 2015 results to use HoC data

[john-sandall]

The currently processed 2015 results data isn't even that nice. Integrate further cleaning and ID mapping from the SixFifty model.

This currently uses the Electoral Commission's results dataset. Earlier this year, they stopped hosting results and this is now officially published by the House of Commons library at parliament.uk. We need to bring this code up-to-date so it uses the new data (which is in a different format!) for the 2015 results: https://researchbriefings.parliament.uk/ResearchBriefing/Summary/CBP-7186

The goal is the same, we want to turn this into a dataset with the same format as the current code with columns something like:

Press Association ID Number    Constituency ID    Constituency Name    Constituency Type    County    Region ID    Region    Country    Election Year    Electorate    Valid Votes    con    lab    ld    ukip    grn    snp    pc    other    con_pc    lab_pc    ld_pc    ukip_pc    grn_pc    snp_pc    pc_pc    other_pc    geo    winner

The new dataset also switches to using just ons_id (which is the same as Constituency ID in the previous cleaned results) and not Press Association ID Number, this is fine, you can drop that column.