Add script to generate flat10-cdr CO2 emissions

[blimlim]

Closes #129

This PR adds a script to generate the CO2 emissions ancillary for the CMIP7 esm-flat10-cdr experiment.

The added arguments for running the script are:

• --year-start: The year to start the ancillary file at. The linear decline in CO2 emissions will start immediately from this point. This should be set to match the year in the flat10 restart file that will be used (currently 301).
• --nyear-zero: Number of years of zero emissions to add at the end. A minimum of 20 years is required, however the experiments could be run for longer. Perhaps 200 years would be suitable.

A test file produced with the script is available at /g/data/tm70/sw6175/development/CMIP7-Input/CMIP7/esm1p6/atmosphere/modern/flat10-cdr/atmosphere/forcing/global.N96/2026.05.13/test_CO2.

The following compares the CO2 emissions in this test file with the ESM1.5 file mentioned in #129.

To test it with the model, I've run several 1 month simulations starting from different years of the esm-flat10 experiment (or the preindustrial+emissions experiment for the higher start years). The following compares the CO2 emissions in the output from these runs with the emissions in the above test ancillary file:

[blimlim]

@penguian, @MartinDix if you are happy to take a look at this, that would be much appreciated!

@penguian once the review process is finished, it would be great to discuss how this can be added to the suite.

[penguian]

Looks good, other than for one comment about PG10_C.

[penguian]

Approved

[blimlim]

As expected, difference against the ESM1.5 file are similar after the rounding change in f745597:

[blimlim]

@penguian, I'm thinking a task which looks something like:

    [[F10CDR_ancil_co2]]
        inherit = ANCIL_CO2
        script = """
            mkdir -p {{ VAR.ANCIL_TARGET_PATH }}
            python_env python \
                -m co2.cmip7_F10CDR_CO2_generate \
                {{ path_args() }}
                {{ grid_args() }}
                --year-start {{ ESM_F10CDR_YEAR_START }}
                --nyear-zero {{ ESM_F10CDR_NYEAR_ZERO }}
                CO2_fluxes_1849_2023_cmip7.anc
                --save-filename {{ quote(ESM_F10CDR_CO2_SAVE_FILENAME) }}
            """

could work for adding this to the suite. The values for the variables would be:

ESM_F10CDR_YEAR_START = 301
ESM_F10CDR_NYEAR_ZERO = 200
ESM_F10CDR_CO2_SAVE_FILENAME = CO2_fluxes_flat10CDR.anc