have datasets return dictionaries

[NickleDave]

because for VOC we want all classes present in addition to largest and random class if required

so for both searchstims and VOC, return dictionary with {'img', 'target'}

additionally for VOC, if largest transform is specified then add 'largest', if 'random' is specified have 'random' ... and so on

also include visual search difficulty score as a key for VOC

[NickleDave]

VOC dict should have

• [ ] classes
• [ ] largest class, if largest class transform not None
• [ ] random class, if random class transform not None
• [ ] visual search difficulty score
• [ ] ind, to get image filename from list if needed