Improve DAIN performance in several aspects

[CyFeng16]

Updates are as follows:

• Improve FFmpeg quality
• Hotfix: Resize post images to minimize the strobe effect, thanks @Brokensilence
• Correct DAIN arguments call
• Finish TODO: Use cv2 to read and write images in loops instead of scipy-1.1.0, python dependencies errors disappear.
• Add some aggressive file check and cleanup to avoid improper use

Tested both locally and Colab(ly).

[CyFeng16]

ops. I should append new README changes instead of reverting this branch, which is too inelegant. sry.