Special handling for flow cytometer images

[metazool]

See #39 for the context - images are 16 bit greyscale but appear in an unusually low range. Here, each is normalised to the maximum pixel value, and for passing through the resnet model is converted into three bands (one per channel, all the same). Shortfall in the PIL library meant we have to do this in an explicit way

• Adds utility functions to normalise and convert the image format
• Adds tests for these and check we can extract embeddings with the outcome
• streamlit demo assumes any 16 bit greyscale image gets this treatment

py.test tests/test_image_embeddings.py