Report validation loss

[rafid-dev]

There's need to check validation loss to know if the net is generalizing well. So I added validation loss reporting

• Add --val-data as the launch argument for passing the validation datasets paths
• Print validation loss after each epoch
• Write the validation to the loss.csv file
• Add two helper functions in dataset: dataset::fetch_dataset_paths for retrieving all files found in directory and dataset::count_total_positions for counting total number of positions given a list of files
• Add --epoch-size as a launch arguments

[jhonnold]

Not everyone uses validation data, this should've been made optional not required.