Update multi-dimentional array example in xray tutorial

[rossbar]

Just something I noticed while reviewing the alt-text changes in the xray image processing tutorial:

The section on loading an "image stack" had a few points that could be improved on. In the first cell in the section, the images are loaded manually rather than programmatically, which is cumbersome and also error prone - for example, there should be 9 images in the final result instead of 8 given the 0-indexed numbering scheme, but img 002 was left out for some reason.

There is also a benchmarking section where appending images in a for-loop is compared to np.stack - however; this is an apple-to-oranges comparison because the former included the file IO in the loop whereas the latter did not. I also think that creating the arrays directly from the data rather than explicitly stacking pre-loaded 1-D arrays demonstrates the multi-dimensional concept more clearly by cutting out unnecessary steps.

[mattip]

Thanks @rossbar