Feature/raw data
Major changes:
- Added in support for plotting raw data
- Moved cache operations to a new Cache class
The way it handles raw data doesn't seem very good, a new method _find_plottable_keys(self) is used to get all keys with a value of type ndarray and assumes that if they have more than three dimensions they are 'plottable, then if image.datahas 4 dimensions it assumes the data is raw, sets a raw flag, and tells the cache to create aimage.gain` entry
Dealing with the cache was getting annoying and there was a bit of duplicate code floating around, so all of the caching is handled by a Cache class now
@kluyvert @tmichela @dallanto @bergeman could you please review when you're free? Thanks