netcdf: add mf6 model

[mjreno]

MODFLOW 6 NetCDF input files from pydantic based model objects:

• "data only" files currently (for simulation input not visualization)
• pyandic NetCDF model objects are internally validated once generated
• Interfaces support creating objects from dictionary descriptions or pyphoenix models
• Interfaces support generating xarray datasets, json schema dictionaries and full meta description dictionaries from objects

Example:

dims = [2, 4, 3, 2]   # "nper", "nlay", "nrow", "ncol" for structured gridtype
recharge = np.linspace(1, 2, 12)
packages = [
    {
        "package_name": "rcha_0",
        "package_type": "gwf-rcha",
        "params": [
            {"name": "recharge", "data": recharge},
        ],
    },
]
nc_cfg = {
    "modeltype": "gwf6",
    "modelname": "gwfmodel",
    "gridtype": "structured",
    "packages": packages,
}

nc_model = NetCDFModel.from_dict(nc_cfg, context={"dims": dims})
ds = nc_model.to_xarray()