Fix error message when reading hspy compressed using blosc

[ericpre]

When reading a hspy file saved using blosc compression, an unclear error message is raised if hdf5plugin hasn't been imported previously - see silx-kit/hdf5plugin#69 (comment).

Progress of the PR

• Try to import hdf5plugin when reading hspy file to avoid unclear error message,
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• ready for review.

Minimal example of the bug fix or the new feature

Create a signal and save it using blosc compression

import hdf5plugin
import hyperspy.api as hs
import numpy as np

s = hs.signals.Signal1D(np.random.random(10*10).reshape((10, 10)))
s.save('test.hspy', compression=hdf5plugin.Blosc(cname='blosclz', clevel=9, shuffle=hdf5plugin.Blosc.SHUFFLE))

Try to open it in a new python process

import hyperspy.api as hs

s = hs.load('test.hspy')

It gives the following error message:

ERROR:hyperspy.io:If this file format is supported, please report this error to the HyperSpy developers.
Traceback (most recent call last):

  File "<ipython-input-3-bed9dafb25ee>", line 1, in <module>
    s = hs.load('test.hspy')

  File "/home/eric/Dev/hyperspy/hyperspy/io.py", line 423, in load
    objects = [load_single_file(filename, lazy=lazy, **kwds) for filename in filenames]

  File "/home/eric/Dev/hyperspy/hyperspy/io.py", line 423, in <listcomp>
    objects = [load_single_file(filename, lazy=lazy, **kwds) for filename in filenames]

  File "/home/eric/Dev/hyperspy/hyperspy/io.py", line 476, in load_single_file
    return load_with_reader(filename=filename, reader=reader, **kwds)

  File "/home/eric/Dev/hyperspy/hyperspy/io.py", line 496, in load_with_reader
    file_data_list = reader.file_reader(filename, **kwds)

  File "/home/eric/Dev/hyperspy/hyperspy/io_plugins/hspy.py", line 180, in file_reader
    exp = hdfgroup2signaldict(exg, lazy)

  File "/home/eric/Dev/hyperspy/hyperspy/io_plugins/hspy.py", line 237, in hdfgroup2signaldict
    data = np.asanyarray(data)

  File "/opt/miniconda3/lib/python3.8/site-packages/numpy/core/_asarray.py", line 171, in asanyarray
    return array(a, dtype, copy=False, order=order, subok=True)

  File "h5py/_objects.pyx", line 54, in h5py._objects.with_phil.wrapper

  File "h5py/_objects.pyx", line 55, in h5py._objects.with_phil.wrapper

  File "/opt/miniconda3/lib/python3.8/site-packages/h5py/_hl/dataset.py", line 1012, in __array__
    self.read_direct(arr)

  File "/opt/miniconda3/lib/python3.8/site-packages/h5py/_hl/dataset.py", line 973, in read_direct
    self.id.read(mspace, fspace, dest, dxpl=self._dxpl)

  File "h5py/_objects.pyx", line 54, in h5py._objects.with_phil.wrapper

  File "h5py/_objects.pyx", line 55, in h5py._objects.with_phil.wrapper

  File "h5py/h5d.pyx", line 192, in h5py.h5d.DatasetID.read

  File "h5py/_proxy.pyx", line 112, in h5py._proxy.dset_rw

OSError: Can't read data (can't open directory: /opt/miniconda3/lib/hdf5/plugin)

[codecov]

Codecov Report

Merging #2760 (522be0b) into RELEASE_next_patch (6d184cd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           RELEASE_next_patch    #2760   +/-   ##
===================================================
  Coverage               77.30%   77.30%           
===================================================
  Files                     202      202           
  Lines                   30032    30036    +4     
  Branches                 6561     6561           
===================================================
+ Hits                    23216    23220    +4     
  Misses                   5068     5068           
  Partials                 1748     1748           

Impacted Files	Coverage Δ
hyperspy/io_plugins/hspy.py	69.01% <100.00%> (+0.26%)	⬆️

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