Parallelize frame processing

[Luni-4]

This PR fixes #10

Starting from the thread-videos branch, I've parallelized the frame processing. Unfortunately, I can't see any notable difference compared to the sequential version. Do you know what could be the problem @shssoichiro?

Thanks in advance for your review! :)

[shssoichiro]

Indeed it does seem to still be using one thread. I'll take a closer look at it later once I get the chance.

[Luni-4]

@shssoichiro

I tried to change the general approach:

• Used par_iter instead of par_bridge because it is more efficient according to the documentation
• Increased the number of threads to 8

But we are slower than the sequential version

[Luni-4]

Wrong approach, so closing it.