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.
• [x] 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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

[](https://app.codecov.io/gh/stefmolin/data-morph/pull/209?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stefanie+Molin) ```diff @@ 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](https://app.codecov.io/gh/stefmolin/data-morph/pull/209?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stefanie+Molin) | Coverage Δ | | |---|---|---| | [src/data\_morph/data/dataset.py](https://app.codecov.io/gh/stefmolin/data-morph/pull/209?src=pr&el=tree&filepath=src%2Fdata_morph%2Fdata%2Fdataset.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stefanie+Molin#diff-c3JjL2RhdGFfbW9ycGgvZGF0YS9kYXRhc2V0LnB5) | `73.07% <100.00%> (ø)` | |