Improved CLI arguments

[KKastberg]

This PR improves the CLI interface by making the default values for the CLI arguments to be retrieved from the config file and adding the following CLI arguments:

• batch_size: Batch size for training
• val_path: Path to val.txt
• log_path: Path to store tensorbordX log files
• checkpoint_path: Path to store checkpoints
• config: Path to a custom yaml config file that overrides cfg.py

The benefit of this PR is that training from CLI is simpler and more versatile. The optional possibility to pass in a custom yaml config file from the command line also makes it great for remote deployment.

I hope you see the benefit and believe it is an improvement. Please contact me with any questions and feel free to edit the code.