Check existence of signal.metadata.Markers before deleting it (was PR #3070)

[nem1234]

Description of the change

• Do not raise AttributeError when loading markers

Progress of the PR

• Change implemented (can be split into several points),
• update docstring (if appropriate),
• update user guide (if appropriate),
• 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

This error does not shown in RELEASE_next_major branch, but potentially raised when broken file reader is loaded.

[codecov]

Codecov Report

Base: 80.60% // Head: 80.64% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (aad704b) compared to base (b7b6f6d).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3091      +/-   ##
======================================================
+ Coverage               80.60%   80.64%   +0.03%     
======================================================
  Files                     176      176              
  Lines                   24476    24474       -2     
  Branches                 5383     5383              
======================================================
+ Hits                    19729    19737       +8     
+ Misses                   3421     3412       -9     
+ Partials                 1326     1325       -1     

Impacted Files	Coverage Δ
hyperspy/io.py	66.56% <75.00%> (+1.04%)	⬆️
hyperspy/signal.py	77.67% <0.00%> (+0.26%)	⬆️

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[ericpre]

I made some comment and it would be good to add a test along the line of:

import hyperspy.api as hs
import numpy as np

s = hs.signals.Signal1D(np.arange(10))
m = hs.plot.markers.Point(x=5, y=5)
s.add_marker(m, permanent=True)

fname = "test.hspy"

s.save(fname)
s2 = hs.load(fname)
assert s.metadata.Markers == ...
s2.plot()

[ericpre]

Suggested change

	Clean up codes around signal.metadata
	Improve syntax in the `io` module.

[ericpre]

Suggested change

	if "Markers" in signal.metadata:
	del signal.metadata.Markers

What's about removing it completely? It is very very rare that we use del before an assignment in python and I don't see why it is necessary here. Looking at the history using git blame, it doesn't seem that there is a reason.