Update for pint unit registry in hyperspy

[ericpre]

See hyperspy/hyperspy#3357 for context

Progress of the PR

• Use pint.get_application_registry instead of HyperSpy private API to get the handle of the pint.UnitRegistry,
• [n/a] docstring updated (if appropriate),
• [n/a] update user guide (if appropriate),
• added tests,
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:holospy build of this PR (link in github checks)
• ready for review.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 86.40%. Comparing base (bd7ade2) to head (e4c3b61).

Files	Patch %	Lines
holospy/signals/hologram_image.py	75.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   87.23%   86.40%   -0.83%     
==========================================
  Files           7        7              
  Lines         329      331       +2     
  Branches       73       74       +1     
==========================================
- Hits          287      286       -1     
- Misses         25       27       +2     
- Partials       17       18       +1     

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