Extend historical aerosol forcings to 1849-2023

[penguian]

See issue #53

The historical period for the aerosol forcings is 1850-2022, but the ancillary files should cover 1849-2023 to allow for interpolation at the start and the end of the time series. As per the nitrogen and ozone forcings, 1849 should be a duplicate of 1850, and 2023 should be a duplicate of 2022.

[penguian]

See also issue #61

[penguian]

The CMIP7 aerosol forcing datasets contain data for 1849 and 2023. Use this instead of duplicating 1850 and 2022.