CHILLED is still using old SSP data and has 2015 as base year

[measrainsey]

As part of the SSP updated process, GDP and population projections have changed, and the new data also has a base year of 2020 (versus currently we're using data with base year 2015). Mia has created gridded versions of the new population data, so we should update CHILLED to use that instead.

Additionally, the base year in CHILLED is still 2015, which is nearly 10 years ago at this point, so it would be good to update to 2020.