Metadata descriptors

[CSSFrancis]

Description of the change

This adds the MetadataAttribute class to hyperspy which maps metadata to an attribute of a class:

Progress of the PR

• Change implemented (can be split into several points),
• update developer guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

class MySignal2D(Signal2D):
    self.title = AttributeDescriptor("General.title")

    def __init__(self,....): 
        # Initialize the signal etc....


s = MySignal2D(np.ones((2,2,2)))

print(s.title) # ""
 s.title = "test"
print(s.title) # "test"
print(s.metadata.General.title) # "test"

#3122 for reference

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a3cf218) 80.81% compared to head (3eedc34) 83.83%.
Report is 816 commits behind head on RELEASE_next_major.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3123      +/-   ##
======================================================
+ Coverage               80.81%   83.83%   +3.02%     
======================================================
  Files                     176      201      +25     
  Lines                   24502    33625    +9123     
  Branches                 5456     9037    +3581     
======================================================
+ Hits                    19802    28191    +8389     
- Misses                   3374     3813     +439     
- Partials                 1326     1621     +295     

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[CSSFrancis]

@jlaehne You might be interested in this as well!

[ericpre]

Re-opening because this has been closed automatically by mistake!