It's a development note. I think that whether or not we compute it must be discussed.

It is only used further in transform and inverse_transform methods. I think that if the user wants it to be computed once and for all, he just has to provide a Memory object. On the other hand, it may be useful for the user to have a direct access to the components in a niimg format. This is more consistent with the behavior of the object : niimg in, niimg out. Maybe we should keep components_img_ and remove components_