Fix Saving Axes in Metadata

[CSSFrancis]

Description of the change

This support saving and loading axes in metadata. Currently this occurs when you find_peaks and it would be good to retain this information.

Progress of the PR

• Change implemented (can be split into several points),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
import numpy as np
s = hs.signals.Signal1D(np.arange(10))
axis = UniformDataAxis(name="x", units="ly", size=10)
s.metadata.set_item("axis", axis)
s.save("temp.hspy")
hs.load("temp.hspy").metadata

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.56%. Comparing base (9b2ed17) to head (5b01cbe).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #3342      +/-   ##
======================================================
+ Coverage               80.51%   80.56%   +0.04%     
======================================================
  Files                     147      147              
  Lines                   21869    21878       +9     
  Branches                 5146     5147       +1     
======================================================
+ Hits                    17607    17625      +18     
+ Misses                   3044     3036       -8     
+ Partials                 1218     1217       -1     

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