updated ROI extraction

Added explicit image creation with individual ROIs and how to extract fMRI data in these ROIs
> In this context it makes sense not to move all the imports at the beginning of
> the scripts but to use them when they are needed. I guess this way it looks
> more like an interactive session (or a tutorial) where the user imports
> packages when he needs them.

Yes. Also when a beginner is reading the script, he can more easily see
the link between the import and how it is used.