Make sure SSP dataset files change at 2015-01-01 rather than 2016

When setting up some datasets for SMYLE I concatenated historical datasets with SSP datasets. I used 2016 as the year to transition to keep the historical period as long as possible. Keith Lindsay would like me to use 2015 rather than 2016. So some of these datasets will need to be changed. Ones to look at are: ndep, presaero, CO2, pop-dens.