TODO: volumetric visualization

[JohnGriffiths]

Have a method in volumetric atlases that uses
nilearn plot_glass_brain to plot tract image
also optionally add roi outlines

Can have the same method for streamlinetric atlases;
generate density map first rather than
extracting image volume

[JohnGriffiths]

made a start on this. need to move it into the viz volumes.py file.