nisl.image.smooth() addition

This function can be used to smooth images (!). It takes niimgs as input, and can process single image or list of images.

The only question is about the naming scheme: I created in the previous branch a "image" package, to contain functions related to niimg processing, as opposed to "signals" which is related to timeseries processing. 

An alternate name would be smooth_img(), but as it is already in the "image" namespace, it seems redundant to me.