Migrate Popolo handling to dedicated library

[jacksonj04]

At the moment the Popolo library replicates many functions of the everypolitician-popolo library, as well as adding its own for manipulating Popolo.

If we plan on using Popolo long term, it would be worthwhile moving write functions to the general purpose library, and then migrating the various scripts which manipulate ParlParse Popolo files to use this.