Rename datasets to data and tidy up submodule

[ericpre]

Shorten hs.datasets.artificial_data.get_wave_image to hs.data.wave_image. Since the eelsdb and other datasets has moved to exspy, we don't need the artificial_data module.

Progress of the PR

• Rename datasets submodule to data and remove artificial_data,
• Tidy up data module
• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de90fee) 80.78% compared to head (6469437) 80.78%.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3253      +/-   ##
======================================================
- Coverage               80.78%   80.78%   -0.01%     
======================================================
  Files                     140      140              
  Lines                   20364    20360       -4     
  Branches                 4820     4820              
======================================================
- Hits                    16451    16447       -4     
  Misses                   2840     2840              
  Partials                 1073     1073              

Files	Coverage Δ
hyperspy/_signals/complex_signal.py	93.75% <ø> (ø)
hyperspy/api_nogui.py	90.00% <ø> (ø)
hyperspy/data/__init__.py	100.00% <100.00%> (ø)
hyperspy/data/artificial_data.py	77.58% <100.00%> (ø)
hyperspy/data/two_gaussians.py	95.83% <ø> (ø)
hyperspy/signal.py	77.52% <ø> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ericpre]

As this is a simple API break, I would suggest to make get it in 2.0!

[CSSFrancis]

LGTM!