Automatically add .mp4 extension to video path
Just a small convenience change:
if video_path has no extension, add .mp4 automatically.
Example: robofish-io-render foo.hdf5 --video-path foo
Same as robofish-io-render foo.hdf5 --video-path foo.mp4 which still works.
The RL rollout tool works in the same way (adds .hdf5 automatically)