Calculate SST box averages

On the assumption that the S8 (10.8┬Ám) nadir brightness temperature variability is a good proxy for the true SST variability, then the difference in temperature between retrieved SST and the S8 nadir BT broadly represents the atmospheric signal. Assuming further that the atmospheric signal is smooth at the resolution of the output grid, then the noise in the atmospheric signal can be reduced by averaging the difference between the retrieved SST and the S8 nadir BT over small scales. An atmospherically smoothed SST can then be derived by adding the average temperature difference back onto the S8 nadir brightness temperature.