New Experiment piClim-histall

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Adding the following new data.

{
    "id": "piclim-histall",
    "type": [
        "wcrp:experiment",
        "cmip7"
    ],
    "activity": [
        "RFMIP"
    ],
    "description": "A simulation with *all* time-varying forcings (*both natural and anthropogenic*) following the historical and ScenarioMIP `scen7-mc` scenario. Greenhouse gases should be prescribed concentrations. Simulations are run atmosphere-only, with a 12-month repeating climatology of sea surface temperatures and sea-ice distributions taken from a climatology of at least 30 years of the same model's `piControl` run. Interactive vegetation, if present in the model, should be turned on. Experiment design is similar to CMIP6 `piClim-histall` experiment (Pincus et al. 2016) with the future scenario changed from `ssp245` to `scen7-mc`.  Pincus, R., Forster, P. M., and Stevens, B.: The Radiative Forcing Model Intercomparison Project (RFMIP): experimental protocol for CMIP6, Geosci. Model Dev., 9, 3447\u20133460, https://doi.org/10.5194/gmd-9-3447-2016, 2016.",
    "label": "piClim-histall",
    "long-label": "historical and future transient effective radiative forcing from all forcers",
    "minimum-number-of-years": "251",
    "model-realms": [
        {
            "id": "agcm",
            "is-required": true
        },
        {
            "id": "aer",
            "is-required": false
        },
        {
            "id": "bgc",
            "is-required": false
        },
        {
            "id": "chem",
            "is-required": false
        }
    ],
    "parent-experiment": [
        "piControl"
    ],
    "start-date": "1850-01-01",
    "sub-experiment": "none",
    "tier": "1"
}

Resolves #18