Enhancement of NeXus file IO

[ptim0626]

Description of the change

The PR contains some improvement and bug fixes about the IO of NeXus file.

• set correct chunk size for lazy dataset (previously it is chunked incorrectly)
• load NeXus dataset and HDF5 dataset consistently
• add option to skip loading array original metadata (this is to avoid duplicating the loading of some large arrays)

hs.load('nexus_file.nxs', skip_array_metadata=True)

• add option to load a specific dataset by specifying absolute path

hs.load('nexus_file.nxs', dataset_path='/entry/instrument/data')

• add option to skip certain original metadata when writing new file by specifying keys

s.save('output.nxs', skip_metadata_key=['sample', 'detector'])

Besides the new options, it also adds tests to cover previously uncovered parts. Any comment is welcomed.

Progress of the PR

• Changes implemented,
• update docstring,
• update user guide,
• add entry to CHANGES.rst,
• add tests,
• ready for review.

[codecov]

Codecov Report

Merging #2725 (2a206a5) into RELEASE_next_minor (9355e30) will increase coverage by 0.25%.
The diff coverage is 93.84%.

@@                  Coverage Diff                   @@
##           RELEASE_next_minor    #2725      +/-   ##
======================================================
+ Coverage               77.75%   78.00%   +0.25%     
======================================================
  Files                     203      203              
  Lines                   30632    30688      +56     
  Branches                 6671     6696      +25     
======================================================
+ Hits                    23817    23938     +121     
+ Misses                   5034     4995      -39     
+ Partials                 1781     1755      -26     

Impacted Files	Coverage Δ
hyperspy/io_plugins/nexus.py	93.62% <93.84%> (+14.37%)	⬆️

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[ericpre]

This looks good to me - see comments.