Update swapper.py

[mst-rajatmishra]

Hello,
I tried making some improvements in code,

• Replaced print statements with logging, which provides better control over the verbosity of output messages.
• face_analyser and face_swapper are loaded only once and reused to avoid redundant loading.
• Added the ability to handle range-based indexes (e.g., 0-2) for source and target faces.
• The ThreadPoolExecutor for possible future parallel processing, though this is not directly applied here but could be added later for batch processing.
• The face restoration part remains the same but is modular and only executed if the flag --face_restore is set.
• Added more informative help text for the command-line arguments.
• More structured handling of swapping faces when using multiple source images, including range checks and more flexible face matching.

Thank You,
Rajat Mishra.