UI/UX: Clarify better the path to the trained_model for metrics evaluation in metrics_config.yaml

[jeipollack]

End users might not fully understand what is required as an entry for the parameter trained_model_path in the metrics_config.yaml file.

  # Fill model_params if computing metrics_only on a pre-trained model else leave blank if training model first
  # Specify path to Trained Model - Optional if model is pre-trained
  trained_model_path: 

They might think they need to provide the full path to the psf_model checkpoint files, rather than just the path to the parent directory (i.e. wf-outputs-YEARMONTHDAYHHMMSS for a previously trained model.  Either rename the parameter or improve the instructions.