Make ImagePlus subscriptable

PyImageJ lets you subscript RandomAccessibleInterval objects, but not ImagePlus objects.

For single-element access:

* GRAY8 (ByteProcessor) is the equivalent of `numpy.uint8`.
* GRAY16 (ShortProcessor) is the equivalent of `numpy.uint16`.
* GRAY32 (FloatProcessor) is the equivalent of `numpy.float32`.
* COLOR_RGB (IntProcessor) [has no numpy dtype equivalent](https://towardsdatascience.com/exploring-the-mnist-digits-dataset-7ff62631766a#a65a) to my knowledge.
* COLOR_256 is apparently [also done in numpy with `numpy.uint8`](https://stackoverflow.com/q/50267606/1207769)...
* Does ImageJ support for uint32 now? I vaguely remember reading on a thread during 2021-2022 about it, but I can't find it now.

Note also that to be consistent with numpy and ImgLib2's behavior, the `dtype` function should also be implemented, and it should return a class object corresponding to the returned type of single elements.

For slicing: maybe we can use the Duplicator's crop and/or run to limit dimensional min/maxes. But step values other than 1 require more effort. Better to wrap the ImagePlus into some kind of Python-side view object?

Start by tracing PyImageJ’s existing RandomAccessibleInterval subscription behavior, then examine ImagePlus and the mentioned Duplicator approach for single-element access, dtype, and slicing. Done means the project has agreed indexing and slicing semantics for ImagePlus, including step values, and dtype returns the corresponding element type; uint32 behavior also needs clarification.

Written by the indexing model from the issue text.