Start work on a utility function to get a dask array from a dataset
variable in a way that is friendly to dask.distributed.

For the distributed-friendly dask array helper, parameterise the test
to cover more cases.  Simplify the implementation.

We need to force the shape and the dtype when getting the
dask-distributed-friendly xarray-dataarray.  Seems to have a first
working prototype now.

Add group support for getting a dask distributed friendly dask array.
Speed up the related tests by sharing the dask distributed client setup
and breakdown.

Add partial backward compatibility for accessing the file handle
attribute when using caching with a NetCDF4FileHandler base class.
Backward incompatibility is not 100%.  Deleting the FileHandler closes
the manager and therefore the ``file_handle`` property, however, when
accessing the ``file_handle`` property after deleting the
``FileHandler``, it is reopened.  Therefore, calling `__del__()``
manually and then accessing ``fh.file_handle`` will now return an open file
(was a closed file).  This should not happen in any sane use scenario.

With the new dask-distributed-friendly caching, make sure we are
respecting auto_maskandscale and are not applying scale factors twice.

Remove a dead code except block that should never be reached.

Migrate TestNetCDF4FileHandler from unittest.TestCase to a regular
class.  Use a pytest fixture for the temporary NetCDF file.

Broaden the string that is matched against in
TestNetCDF4FileHandler.test_filenotfound.  On Linux and MacOS the
expected failure gives "No such file or directory".  On Windows it gives
"Invalid file format".

Fix the spelling in the docstring example using netCDF4.

Co-authored-by: David Hoese <[email protected]>

Add a workaround to prevent an unexpected type promotion in the unit
test for dask distributed friendly dask arrays.

When getting a dask-distributed friendly dask array from a NetCDF file
using the CachingFileManager, use the information provided in bloc_info
on the array location in case we are reading not the entire variable.

Rename get_distributed_friendly_dask_array to
get_serialisable_dask_array and remove the group argument, moving the
responsibility for handlings groups to the caller.

Pytroll uses US spelling.  Rename serializable to serialisable.

Remove removed keyword argument from call.

Ensure that the meta we pass to map_blocks also has the right dtype.
Not sure if this is necessary when map_blocks already has the right
dtype, but it can't hurt.

Fixing three merge conflicts.

When caching, make sure we use the CachingFileManager already upon scene
creation and not only by the time we are loading.

Don't subclass netCDF4.Dataset, rather just return an instance from a
helper function.  Seems good enough and gets rid of the weird error
messages upon exit.

Some readers read entire groups; this needs xarray kwargs to be set even
if caching is used.