updated ROI extraction

Added explicit image creation with individual ROIs and how to extract fMRI data in these ROIs
I would avoid explicitly specifying default parameter values. It makes for a simpler example and gives us more leeway to change defaults in the future if we wish to do so.

From a quick look at the code it seems to be the case for:
* `background_label`
* `smoothing_fwhm`
* `verbose`