add pretrained models

[kingfischer16]

Create a pretrained folder with a library of models. First should be the SRCNN model as found in the paper, with matching benchmark scores. Possibly add ensemble models, models trained with other loss functions, augmented data, or alternative datasets (with scores as well).

[kingfischer16]

pretrained SRCNN model (basic plus some of the better variants) were added in the #2 PR.