Add `RawImage.split()` function to split images into channels; Improved documentation and tests (#978)
* Add tests for original slice method.
* Add vslice and tests to retrieve the entire length of a column.
* Add a test for slicing every other column.
* Add method to return each channel as a separate array.
* Add documentation.
Fix TypeScript error for unsure type.
* Remove vslice as it doesn't work as it should.
Update documentation.
Update tests.
* Optimize `RawImage.split()` function
* Use dummy test image
* Update tensor unit tests
* Wrap `.split()` result in `RawImage`
* Update JSDoc
* Update JSDoc
* Update comments
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Co-authored-by: Joshua Lochner <admin@xenova.com>