Fix finding dataset in group in EMD NCEM file

[ericpre]

Fixes #2614.

Progress of the PR

• Fix bug with finding dataset path for (non-complicant?) file containing several dataset in a group,
• add entry to CHANGES.rst (if appropriate),
• ready for review.

[codecov]

Codecov Report

Merging #2673 (8015984) into RELEASE_next_patch (240bf42) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           RELEASE_next_patch    #2673   +/-   ##
===================================================
  Coverage               76.62%   76.62%           
===================================================
  Files                     201      201           
  Lines                   29681    29683    +2     
  Branches                 6500     6501    +1     
===================================================
+ Hits                    22743    22745    +2     
  Misses                   5177     5177           
  Partials                 1761     1761           

Impacted Files	Coverage Δ
hyperspy/io_plugins/emd.py	66.70% <100.00%> (+0.07%)	⬆️

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[jlaehne]

LGTM