Potential bug when computing average color from mask

Only the value in the green channel is returned here:

https://gitlab.idiap.ch/bob/bob.rppg.base/blob/master/bob/rppg/cvpr14/extract_utils.py#L296

But this is used to compute the pulse in all color channels, as in here:

https://gitlab.idiap.ch/bob/bob.pad.face/blob/master/bob/pad/face/preprocessor/Li.py#L151