Correct detector shape of returned EBSD signal from downsample()

[hakonanes]

Description of the change

In preparation for a workshop during the two-days Nordic Nanolab User Meeting (NNUM) in Oslo next week, I encountered some bugs and things which need updating.

Fixes #668.

Progress of the PR

• [x] Docstrings for all functions
• [x] Unit tests with pytest for all lines
• [x] Clean code style by running black via pre-commit

For reviewers

• [ ] The PR title is short, concise, and will make sense 1 year later.
• [ ] New functions are imported in corresponding __init__.py.
• [ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
• [ ] New contributors are added to release.py, .zenodo.json and .all-contributorsrc with the table regenerated.

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