Add n images

[yw7]

Add code for:

• Add n_images argument to generate_image.py to enable generating multiple images at once.
• Remove femur label.
• Add output_bids argument to combine_masks.py to enable output not in BIDS structure so we can combine TotalSeg dataset labels and use folder containing all segmentations as input to BrainGenerator.
• Add default-stats argument to extract_signal_statistics.py to enable setting the default values for signal statistics if label does not exists in the input images.