stack raises error when stacking signals that contain a variance signal

[francisco-dlp]

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

The following raises an error without this fix:

import hyperspy.api as hs
import numpy as np

s = hs.signals.EELSSpectrum(np.random.random((10, 10, 100)))
s.axes_manager[0].name = "x"
s.axes_manager[1].name = "y"
s.estimate_poissonian_noise_variance()
hs.stack([s, s], axis="y")

[codecov]

Codecov Report

Merging #2954 (c66daa3) into RELEASE_next_patch (3634a16) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #2954      +/-   ##
======================================================
- Coverage               80.87%   80.80%   -0.07%     
======================================================
  Files                     209      209              
  Lines                   32654    32698      +44     
  Branches                 7313     7322       +9     
======================================================
+ Hits                    26408    26421      +13     
- Misses                   4483     4514      +31     
  Partials                 1763     1763              

Impacted Files	Coverage Δ
hyperspy/misc/utils.py	91.00% <ø> (+0.46%)	⬆️
hyperspy/axes.py	91.18% <100.00%> (+0.01%)	⬆️
hyperspy/samfire_utils/samfire_worker.py	65.42% <0.00%> (-19.16%)	⬇️
hyperspy/misc/eels/base_gos.py	35.00% <0.00%> (-1.59%)	⬇️
hyperspy/model.py	83.80% <0.00%> (-0.21%)	⬇️
hyperspy/interactive.py	86.79% <0.00%> (ø)
hyperspy/_signals/eds.py	85.56% <0.00%> (ø)
hyperspy/_signals/eels.py	77.43% <0.00%> (ø)
hyperspy/misc/elements.py	100.00% <0.00%> (ø)
hyperspy/_signals/eds_sem.py	60.60% <0.00%> (ø)
... and 12 more

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[jlaehne]

otherwise, lgtm

[ericpre]

Other than adding a test, as mentioned by @jlaehne, it looks good!