Enable single and paired datasets validation. Add dataset specific image saving options

[marcobarbierato]

nondist_validation now first checks dataset type, and if it's single disables metric computations (with_metrics). That is enough to use both single and paired dataset in validation, e.g. if one wants to use the paired only for metrics and the single one for a visual check.

Adds support for two dataset options.

1. save_img if present overwrites the global setting for saving images to disk.
2. save_tb if true saves the image in tensorboard for visualization.

There is no need to modify existent options.