Reading the SEVIRI HRV channel with seviri_l1b_native returns a numpy array

[ameraner]

Describe the bug
The seviri_l1b_native reader returns a numpy array instead of a dask array for the HRV channel. Apart from the reader not being dask-compliant, another symptom of this is Satpy failing on e.g. loading composites combining HRV and channels with other resolutions.

To Reproduce

from satpy.scene import Scene

scn_sev = Scene(filenames=[sev_filepath], reader='seviri_l1b_native')

scn_sev.load(['HRV'])
print(type(scn_sev['HRV'].data))
# prints: <class 'numpy.ndarray'>

scn_sev.load(['vis_sharpened_ir'])

Expected behavior
A correctly loaded dataset and HRV channel data in a dask array.

Actual results

Traceback (most recent call last):
  File "/tcenas/home/andream/anaconda3/envs/trolldev_latest/lib/python3.7/site-packages/IPython/core/interactiveshell.py", line 3343, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-1-4de67669f23a>", line 1, in <module>
    scn_sev.load(['vis_sharpened_ir'])
  File "/tcenas/home/andream/code/satpy_latest/satpy/satpy/scene.py", line 975, in load
    keepables = self.generate_composites()
  File "/tcenas/home/andream/code/satpy_latest/satpy/satpy/scene.py", line 888, in generate_composites
    return self._read_composites(nodes)
  File "/tcenas/home/andream/code/satpy_latest/satpy/satpy/scene.py", line 863, in _read_composites
    self._generate_composite(item, keepables)
  File "/tcenas/home/andream/code/satpy_latest/satpy/satpy/scene.py", line 838, in _generate_composite
    **self.attrs)
  File "/tcenas/home/andream/code/satpy_latest/satpy/satpy/composites/__init__.py", line 398, in __call__
    lons, lats = vis.attrs["area"].get_lonlats(chunks=vis.data.chunks)
AttributeError: 'numpy.ndarray' object has no attribute 'chunks'

[ameraner]

One hurdle for dask-ifying the reader is implementing the channel lines interleaving that is necessary to correctly read the native data (see these lines). @mraspaud suggests something like:
data = hstack((data0, data1, data2)).reshape(data0.shape[0], :)

[ameraner]

FYI, even after #1272 is merged, scn_sev.load(['vis_sharpened_ir']) will still fail due to mismatching chunk sizes. But this is a bug in pyresample and not related to the SEVIRI native reader (see Slack discussion)

[ameraner]

The issue I mention in the last message was fixed in pytroll/pyresample#313