Extend historical volcanic forcing to 2300

[penguian]

In https://github.com/ACCESS-NRI/UM7/blob/dev-access-esm1.6/umbase_hg3/src/control/top_level/setcona.F90 there occur the following lines:

      if (L_VOLCTS) then
      write(6,*) 'SETCONA: Reading in volcanic forcing from file: ',    &
     &            FILE_VOLCTS
      OPEN(UNIT=56,file=FILE_VOLCTS,status='old') 
      do i=1850,2300 
        do j=1,12 
          read(56,*) yvolc,mvolc,volcts(:,j,i) 
        enddo 
      enddo 
      CLOSE(56)
      endif

Therefore the historical volcanic forcing file needs to contain values representing each month of the years 1850 to 2300. For the historical experiment, the values after CMIP7_HI_VOLCANIC_END_YEAR (2023) can be dummy values. The dummy values could just be a repeat of the 1850 values.

[MartinDix]

For CMIP6 the recommendation for future scenarios was to use the historical mean (i.e. the PI control value) from 2025 and to linearly interpolate between 2015 and 2025.

[penguian]

The code now linearly interpolates between 2023 and CMIP7_HI_SAOD_TAPER_END_YEAR = 2033.

[MartinDix]

ScenarioMIP paper https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3765/ doesn't say anything about solar or volcanic forcing. We might have to revist this.