Kodak24 testing setting may lead to unfair comparison

[Schizophreni]

For DnCNN and FFDNet, the images in Kodak24 are center cropped into 500x500 for testing (Those images are from https://github.com/cszn/FFDNet/tree/master/testsets/Kodak24). However, the Kodak24 testing set provided in download_data.py does not follow that setting, which may result in an unfair comparison.