netcdf input utilities and validation

[mjreno]

dfn branch version of #260

from modflow_devtools.netcdf import (
    NetCDFModelInput,
    NetCDFPackageCfg,
)

nc_input = NetCDFModelInput(
    name="twri",
    type="gwf",
    grid_type="structured",
    mesh_type="layered",
    dims=[2, 4, 6],  # ["time", "z", "nmesh_face"]
)

nc_input.packages.append(NetCDFPackageCfg("npf", "npf", params=["k", "k22"]))
nc_input.packages.append(NetCDFPackageCfg("welg_0", "welg", params=["q"]))

ds = nc_input.to_xarray()