axes fix for loading JPG with no exif_tags

[ThorstenBASF]

Description of the change

A few sentences and/or a bulleted list to describe and motivate the change:

• JPG files with no exif_tags had no axes or dimensions when loaded.
• instead of returning empty axes, axes are returned with an scale of 1 pixel/pixel.
• with fix this images can be handled with hs/axes commands as usual. (s.calibrate e.g.)

Progress of the PR

• Change implemented (can be split into several points),
• update docstring (if appropriate),
• update user guide (if appropriate),
• 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:rosettasciio build of this PR (link in github checks)
• add tests,
• ready for review.

---

• add testfile

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
tfile = "./rsciio/tests/data/image/jpg_no_exif_tags.jpg"
s = hs.load(tfile)
s.calibrate(x0=0, y0=0, x1=0, y1=1, new_length=0.2, units="nm", interactive=False)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.52%. Comparing base (de98a8e) to head (2b38d85).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   86.33%   86.52%   +0.18%     
==========================================
  Files          83       83              
  Lines       10674    10676       +2     
  Branches     2332     2332              
==========================================
+ Hits         9215     9237      +22     
+ Misses        940      922      -18     
+ Partials      519      517       -2     

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[ericpre]

Thanks, this looks good, I left two comments.

[ericpre]

In this case, the standard pattern is to set to None and in hyperspy will be parsed a traits.api.undefined value.

Suggested change

	unit = "pixel"
	unit = None

[ericpre]

While you are at it and in the interest of readability, can you please rename here and in the two other places below to

Suggested change

	scales = [sizes[1],sizes[0]]
	fiels_of_views = [sizes[1],sizes[0]]

[ThorstenBASF]

Thanks, did the changes, also set unit to "None".

[ericpre]

pre-commit.ci autofix