fix load_png_images to use bounding box starts & stops

[eschombu]

I fixed the ability to load pngs from smaller chunks. Note that this is not very speed efficient, because each chunk task will load the number of pngs specified by the chunk size in the z dimension (e.g., 128), but at least it will not store the full pngs in memory, only the portions specified by the x, y chunk dimensions.

[eschombu]

@jingpengw, I added the conftest.py file to get pytest to load the chunkflow module and run all the tests during the github actions.