Adapt transform to read in set of specified datasets and run the code for each

[soniwe]

In "Loading the data", we need to be able to specify a set of files (using the DHS labelling system), and make this code run over all of them. At the end of the transforming process, do we need to create new reduced file for each country/year, labelled with the country and year in the title - e.g. AFG2015, for Afghanistan 2015.