updated ROI extraction

Added explicit image creation with individual ROIs and how to extract fMRI data in these ROIs
Although I would definitely agree in general, my impression is that these example scripts are written in order to be easily separated into stand-alone snippets that can be easily inserted into the documentation. For example this script is used as part of the documentation [here](https://nilearn.github.io/building_blocks/manipulating_mr_images.html#mask-dilation).

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.