Describe your changes
Calls to read_image by functions in the visualize and transform subpackages create circular imports, because read_image imports the hyperspectral subpackage. This PR resolves some circular imports by:

• Creating the helper function _read_hyper in readimage, taking over functionality from hyperspectral.read_data.
• Relocating helper functions in hyperspectral.read_data (that are only called by read_data) into the readimage module.
• Redirects hyperspectral.read_data to wrap _read_hyper, to preserve existing workflows.
• Relocating shared helper functions _find_closest and _make_pseudo_rgb from hyperspectral.read_data to the _helpers module.
• Creating a helper for transform.rescale in _helpers.

Type of update
Code cleanup

For the reviewer
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• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved