Add support for chunked arrays and dask operations

[tomsail]

The current indexing induces problems when doing dask operations on chunked arrays:

Although the chunking does not raise any Error:

import xarray as xr

file = "tests/data/r3d_bump.slf"
ds = xr.open_dataset(file)
print(ds)
ds = ds.chunk({"time": -1, "node": 40})

def analyze_block(ds_block: xr.Dataset) -> xr.Dataset:
    """Operate on a single Dask chunk."""
    result = ds_block.mean(dim="node")
    return result

result = xr.map_blocks(analyze_block, ds)
result.compute()

The actual call to the underlying data:

print(result.Z.values)

raises:

Traceback (most recent call last):
  File "./test_dask.py", line 19, in <module>
    result.compute()
  File "./.venv/lib/python3.12/site-packages/xarray/core/dataset.py", line 791, in compute
    return new.load(**kwargs)
           ^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/dataset.py", line 557, in load
    evaluated_data: tuple[np.ndarray[Any, Any], ...] = chunkmanager.compute(
                                                       ^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/namedarray/daskmanager.py", line 85, in compute
    return compute(*data, **kwargs)  # type: ignore[no-untyped-call, no-any-return]
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/dask/base.py", line 681, in compute
    results = schedule(expr, keys, **kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/indexing.py", line 659, in __array__
    return np.asarray(self.get_duck_array(), dtype=dtype, copy=copy)
                      ^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/indexing.py", line 664, in get_duck_array
    return self.array.get_duck_array()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/indexing.py", line 943, in get_duck_array
    duck_array = self.array.get_duck_array()
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/indexing.py", line 897, in get_duck_array
    return self.array.get_duck_array()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/indexing.py", line 737, in get_duck_array
    array = self.array[self.key]
            ~~~~~~~~~~^^^^^^^^^^
  File "./xarray_selafin/xarray_backend.py", line 187, in __getitem__
    return indexing.explicit_indexing_adapter(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/xarray/core/indexing.py", line 1129, in explicit_indexing_adapter
    result = raw_indexing_method(raw_key.tuple)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./xarray_selafin/xarray_backend.py", line 246, in _raw_indexing_method
    temp = np.reshape(temp, (self.shape[1], self.shape[2]))  # (nplan, nnode)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/numpy/_core/fromnumeric.py", line 324, in reshape
    return _wrapfunc(a, 'reshape', shape, order=order)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./.venv/lib/python3.12/site-packages/numpy/_core/fromnumeric.py", line 57, in _wrapfunc
    return bound(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^^
ValueError: cannot reshape array of size 3 into shape (5,1452)

This is because Dask might pass keys like (0,), (slice(0,1), slice(0,50)), or even (slice(None), slice(0,50)).
In those cases, the _raw_indexing_method logic doesn’t match