[Feature] Clean up `brainglobe_registration.utils.utils.py`

The `calculate_rotated_bounding_box` function better belongs in `brainglobe_registration.utils.transforms.py` rather than the generic `utils`.

The generic `utils` file could be refactored completely into a `brainglobe_registration.utils.file.py`, and `brainglobe_registration.utils.napari.py` to improve organisation.