Add file saving functionality to plots and metrics

• Modified plot.py to include saving SHAP feature importance and segment views as PNG files in an out directory.
• Adjusted the plot_feature_importance and plot_segment_views functions to accept additional parameters for naming saved files.
• Updated train.py to print the count of found NIfTI files and adjusted the TSV file path generation.
• Implemented saving detailed accuracy metrics to a text file in the out directory, while still printing to console for immediate feedback.
• Ensured all directories are created if they don't exist before saving outputs.