Resampling not working with SLSTR ancillary datasets

[simonrp84]

Describe the bug
SLSTR dataset contains both channel data and ancillary data such as viewing geometry. When resampling a scene, this viewing geometry is not resampled correctly.

To Reproduce

from satpy import Scene
from glob import glob

infiles = glob('/my_slstr_dir/timeslot.SEN3/*.nc')
scn = Scene(infiles, reader='slstr_l1b')

scn.load(['S1', 'satellite_azimuth_angle'])
scn2 = scn.resample('laea_bb', resampler='bilinear')
data = np.array(scn4['satellite_azimuth_angle'])

Expected behavior
The azimuth angles on the resampled grid to be stored in the data variable.

Actual results

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-17-2c88e77567dd> in <module>
----> 1 np.array(scn4['satellite_azimuth_angle'])

~/miniconda3/lib/python3.8/site-packages/xarray/core/common.py in __array__(self, dtype)
    139 
    140     def __array__(self: Any, dtype: DTypeLike = None) -> np.ndarray:
--> 141         return np.asarray(self.values, dtype=dtype)
    142 
    143     def __repr__(self) -> str:

~/miniconda3/lib/python3.8/site-packages/xarray/core/dataarray.py in values(self)
    632     def values(self) -> np.ndarray:
    633         """The array's data as a numpy.ndarray"""
--> 634         return self.variable.values
    635 
    636     @values.setter

~/miniconda3/lib/python3.8/site-packages/xarray/core/variable.py in values(self)
    518     def values(self):
    519         """The variable's data as a numpy.ndarray"""
--> 520         return _as_array_or_item(self._data)
    521 
    522     @values.setter

~/miniconda3/lib/python3.8/site-packages/xarray/core/variable.py in _as_array_or_item(data)
    260     TODO: remove this (replace with np.asarray) once these issues are fixed
    261     """
--> 262     data = data.get() if isinstance(data, cupy_array_type) else np.asarray(data)
    263     if data.ndim == 0:
    264         if data.dtype.kind == "M":

~/miniconda3/lib/python3.8/site-packages/numpy/core/_asarray.py in asarray(a, dtype, order, like)
    100         return _asarray_with_like(a, dtype=dtype, order=order, like=like)
    101 
--> 102     return array(a, dtype, copy=False, order=order)
    103 
    104 

~/miniconda3/lib/python3.8/site-packages/dask/array/core.py in __array__(self, dtype, **kwargs)
   1500 
   1501     def __array__(self, dtype=None, **kwargs):
-> 1502         x = self.compute()
   1503         if dtype and x.dtype != dtype:
   1504             x = x.astype(dtype)

~/miniconda3/lib/python3.8/site-packages/dask/base.py in compute(self, **kwargs)
    283         dask.base.compute
    284         """
--> 285         (result,) = compute(self, traverse=False, **kwargs)
    286         return result
    287 

~/miniconda3/lib/python3.8/site-packages/dask/base.py in compute(*args, **kwargs)
    565         postcomputes.append(x.__dask_postcompute__())
    566 
--> 567     results = schedule(dsk, keys, **kwargs)
    568     return repack([f(r, *a) for r, (f, a) in zip(results, postcomputes)])
    569 

~/miniconda3/lib/python3.8/site-packages/dask/threaded.py in get(dsk, result, cache, num_workers, pool, **kwargs)
     77             pool = MultiprocessingPoolExecutor(pool)
     78 
---> 79     results = get_async(
     80         pool.submit,
     81         pool._max_workers,

~/miniconda3/lib/python3.8/site-packages/dask/local.py in get_async(submit, num_workers, dsk, result, cache, get_id, rerun_exceptions_locally, pack_exception, raise_exception, callbacks, dumps, loads, chunksize, **kwargs)
    512                             _execute_task(task, data)  # Re-execute locally
    513                         else:
--> 514                             raise_exception(exc, tb)
    515                     res, worker_id = loads(res_info)
    516                     state["cache"][key] = res

~/miniconda3/lib/python3.8/site-packages/dask/local.py in reraise(exc, tb)
    323     if exc.__traceback__ is not tb:
    324         raise exc.with_traceback(tb)
--> 325     raise exc
    326 
    327 

~/miniconda3/lib/python3.8/site-packages/dask/local.py in execute_task(key, task_info, dumps, loads, get_id, pack_exception)
    221     try:
    222         task, data = loads(task_info)
--> 223         result = _execute_task(task, data)
    224         id = get_id()
    225         result = dumps((result, id))

~/miniconda3/lib/python3.8/site-packages/dask/core.py in _execute_task(arg, cache, dsk)
    119         # temporaries by their reference count and can execute certain
    120         # operations in-place.
--> 121         return func(*(_execute_task(a, cache) for a in args))
    122     elif not ishashable(arg):
    123         return arg

~/miniconda3/lib/python3.8/site-packages/pyresample/bilinear/xarr.py in _delayed_slice_data(self, slicer, data, fill_value)
    157     @delayed(nout=4)
    158     def _delayed_slice_data(self, slicer, data, fill_value):
--> 159         return slicer(data.values, self.slices_x, self.slices_y, self.mask_slices, fill_value)
    160 
    161     def _get_target_proj_vectors(self):

~/miniconda3/lib/python3.8/site-packages/pyresample/bilinear/_base.py in _slice2d(values, sl_x, sl_y, mask, fill_value)
    626 def _slice2d(values, sl_x, sl_y, mask, fill_value):
    627     # Slice 2D data
--> 628     arr = values[(sl_y, sl_x)]
    629     arr[(mask, )] = fill_value
    630     return arr[:, 0], arr[:, 1], arr[:, 2], arr[:, 3]

IndexError: index 1200 is out of bounds for axis 0 with size 1200

[simonrp84]

Note:
I've also tried with ewa resampling, which crashes python - I'm not sure why, and nearest and gradient_search, which both give the following error:

~/miniconda3/lib/python3.8/site-packages/dask/array/core.py in normalize_chunks(chunks, shape, limit, dtype, previous_chunks)
   2803             for c, s in zip(map(sum, chunks), shape)
   2804         ):
-> 2805             raise ValueError(
   2806                 "Chunks do not add up to shape. "
   2807                 "Got chunks=%s, shape=%s" % (chunks, shape)

ValueError: Chunks do not add up to shape. Got chunks=((512, 512, 176), (512, 512, 476)), shape=(2400, 3000)