Design access mechanism, dataset types for accessing flats from Butler

Need to be able to retrieve dithered flats, aperture definition flats (equivalent to dither=0), etc. One option is to load all flats from the Butler and sort them out in Python code; another is to define separate dataset types, or do something clever with DataIds. Please:

• Write down all the different types of flats you expect to be working with;
• Produce a straw-man design for how you want to organize them.