Converting frame containers to numpy arrays are useful for everyone.

I can see that why a function which takes a frame container and converts it to a numpy array could be useful here but it is also useful in all packages that use bob.bio.video. Hence, this is better to be implemented in bob.bio.video.

https://gitlab.idiap.ch/bob/bob.pad.face/blob/b0a14393f109e8bc15928ade60b0614e34e4b73f/bob/pad/face/algorithm/VideoSvmPadAlgorithm.py#L116