Refactor data loading in `train.py` for memory efficiency
- Replace lists of volumes and labels with DataFrames for efficient concatenation.
- Update loop to process each NIfTI file individually, freeing memory after processing.
- Use index of file in loop to aide in processing.
- Modify how the source file is tracked within the DataFrame.
- Correct plotting function to use a single sample volume instead of combined volumes.
- Ensure proper handling of variables for most important segment visualization.