non-uniform updates (NotImplementedErrors, tests, spelling, saving-fix)

[jlaehne]

Description of the change

• Replace remaining mentions of 'non-linear' instead of 'non-uniform' axis
• Replace remaining misspellings of 'non uniform' instead of 'non-uniform' axis
• Add missing catches for non-uniform axis few sentences and/or a bulleted list to describe and motivate the change:
• Implement interpolate_in_between for non-uniform axis with delta being float
• fix error in io-save-code introduced by some merge of RnM: https://github.com/hyperspy/hyperspy/pull/2399/files#r629224770

Progress of the PR

• Changes implemented (can be split into several points),
• add tests,
• ready for review.

[codecov]

Codecov Report

Merging #2761 (6d4de2a) into non_uniform_axes (516b193) will increase coverage by 0.04%.
The diff coverage is 89.47%.

@@                 Coverage Diff                  @@
##           non_uniform_axes    #2761      +/-   ##
====================================================
+ Coverage             77.90%   77.95%   +0.04%     
====================================================
  Files                   203      203              
  Lines                 31166    31136      -30     
  Branches               6807     6804       -3     
====================================================
- Hits                  24280    24271       -9     
+ Misses                 5085     5064      -21     
  Partials               1801     1801              

Impacted Files	Coverage Δ
hyperspy/component.py	85.61% <ø> (ø)
hyperspy/datasets/artificial_data.py	98.64% <ø> (ø)
hyperspy/io_plugins/hspy.py	68.60% <ø> (ø)
hyperspy/models/model1d.py	87.96% <ø> (ø)
hyperspy/_signals/signal1d.py	77.73% <80.00%> (+0.30%)	⬆️
hyperspy/io.py	84.67% <100.00%> (+6.69%)	⬆️
hyperspy/io_plugins/emd.py	66.95% <100.00%> (ø)
hyperspy/io_plugins/jeol.py	92.28% <100.00%> (+0.02%)	⬆️
hyperspy/signal_tools.py	69.02% <100.00%> (+0.15%)	⬆️
hyperspy/io_plugins/msa.py	75.75% <0.00%> (-1.22%)	⬇️
... and 3 more

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[ericpre]

Looks good to me - I added suggestions for improvement to make it slightly more readable.