The current code will always extract all of the input video frames.
This PR adds two new input parameters for extracting an input video:
extract from frame, and extract to frame.
Now we can only test frames 0 to 50, 50 to 100, etc.
better handling of wrong file/folder names, added function to delete all imgs from a folder (shorter code); now users can generate only the masks again even without having the 'generate_hybrid_frames' button selected