Add get_image and get_image_label

[eteq]

As discussed with @mwcraig at the coordination meeting. For now we are setting no type limitations on the returned image since backends often have different internal representations.

Edit: fixes #59

[mwcraig]

Thanks -- comments are inline.

[mwcraig]

Interesting; logically I would have thought of this as being 1.

Is this too complicated: None only counts as an image label if it has data associated with it

So the idea would be that if, like in this example, the only loaded data was for an image label you set, the get_image_labels returns just the image label you have used.

Doing this would return two image labels:

# Assume we are starting with a clean slate

assert self.image.get_image_labels() is None  # or maybe == []

# Load an image without a label, which means the label is `None`
self.image.load_image(data)

# Load a second image with an explicit label
self.image.load_image(data, label="test")

assert len(self.image.get_image_labels()) == 2  # None and "test"

[mwcraig]

Tracking this in #74