add CF's style _FillValue attribute at array level #171

import zarr
import xarray
import numpy
from xarray.backends.zarr import FillValueCoder

store = zarr.storage.MemoryStore()
dataset = zarr.create_group(store=store)
array = dataset.create_array(
    "data",
    shape=(1800, 3600),
    chunks=(50, 50),
    dtype="float32",
    fill_value=-9999.0,
    dimension_names=("y", "x"),
    attributes={
        "_FillValue": FillValueCoder.encode(-9999.0, numpy.dtype("float32")),
    },
)
array[:] = numpy.arange(0.0, 3600 * 1800, dtype="float32").reshape(1800, 3600)
array[0:50, 0:50] = -9999.0  # Add some nodata
zarr.consolidate_metadata(dataset.store)

ds = xarray.open_dataset(
    store,
    decode_times=False,
    decode_coords=False,
    consolidated=True,
    use_zarr_fill_value_as_mask=True,
    engine="zarr",
)

ds["data"].to_masked_array()
>> masked_array(
  data=[[--, --, --, ..., 3597.0, 3598.0, 3599.0],
        [--, --, --, ..., 7197.0, 7198.0, 7199.0],
        [--, --, --, ..., 10797.0, 10798.0, 10799.0],
        ...,
        [6469200.0, 6469201.0, 6469202.0, ..., 6472797.0, 6472798.0,
         6472799.0],
        [6472800.0, 6472801.0, 6472802.0, ..., 6476397.0, 6476398.0,
         6476399.0],
        [6476400.0, 6476401.0, 6476402.0, ..., 6479997.0, 6479998.0,
         6479999.0]],
  mask=[[ True,  True,  True, ..., False, False, False],
        [ True,  True,  True, ..., False, False, False],
        [ True,  True,  True, ..., False, False, False],
        ...,
        [False, False, False, ..., False, False, False],
        [False, False, False, ..., False, False, False],
        [False, False, False, ..., False, False, False]],
  fill_value=np.float64(1e+20),
  dtype=float32)