The download endpoint now provides a netcdf file download for a pollutant. The download for a pollutant is either taken from pre-prepared data or generated dynamically. Pre-prepared data is used when a user requests a download for either Total Emissions or for a Snap Sector. A dynamic download is prepared using raw Ricardo activity grids and emission factors when the user requests data that includes sub-Snap-Sector options - which is currently only for Road Transport (things like vehicle type and driving activity type). Additional endpoints have been added to provide data to dropdowns on initialisation. This allows easy changing of data as new pollutants and snap sectors are added.