Default value for ATM_ALT_CO2_OPT should be const

[mnlevy1981]

Just need

    ATM_ALT_CO2_OPT:
        description: |
            "default comes from env_run.xml (OCN_CO2_TYPE)
             Source of atmospheric CO2 concentration for MARBL.
             valid values: constant, prognostic, diagnostic"
        datatype: string
        value:
-           '"BASE_BIO_ON=TRUE" in $MARBL_TRACER_OPTS': $OCN_CO2_TYPE
+           '"BASE_BIO_ON=TRUE" in $MARBL_TRACER_OPTS': constant

and then running the script to update the JSON as well; I think this will be bit-for-bit in most G compsets because OCN_CO2_TYPE = constant unless we use a compset with CAM, DATM%CPLHIST, OMIP_DATM or _BGC%[BDRD|BPRP]. There are some CPLHIST compsets, but no G compsets with _BGC or the OMIP time period (yet). It will be answer-changing in fully-coupled runs, but Cecile is currently making this change via user_nl_mom

[mnlevy1981]

The default in the fortran code is constant, so we don't need to include this variable in the YAML file at all:

    call get_param(param_file, mdl, "ATM_ALT_CO2_OPT", atm_co2_opt, &
        "Source of alternate atmospheric CO2 [constant, diagnostic, or prognostic]", &
        default="constant")