Fix return type should be BoundingBox

[robvdl]

The return type of these three methods is set to None, but it returns a BoundingBox:

• _derive_data_bounds
• _derive_morphing_bounds
• _derive_plotting_bounds

Set the correct return type to BoundingBox instead of None.

Fixes #208

• Test cases have been modified/added to cover any code changes.
• Docstrings have been modified/created for any code changes.
• All linting and formatting checks pass (see the contributing guidelines for more information).

I'm not sure if test cases are needed because this change is so small.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.41%. Comparing base (51b8d26) to head (f52c5d3).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
- Coverage   98.42%   98.41%   -0.01%     
==========================================
  Files          43       43              
  Lines        1775     1771       -4     
  Branches      358      357       -1     
==========================================
- Hits         1747     1743       -4     
  Misses         25       25              
  Partials        3        3              

Files	Coverage Δ
src/data_morph/data/dataset.py	73.07% <100.00%> (ø)

[stefmolin]

Thanks, @robvdl!