MRG: API to help modify image caching

Add 'caching='fill'|'unchanged' keyword to ``get_data`` method.

Add ``in_memory`` property to tell user whether image already has an in-memory copy of the data.
I guess I could just be a very naive user, but I cannot figure out what this paragraph is telling me--it seems terribly complicated!  I have no idea how I would go about using this flag correctly.