Lower boundary doesn't work for mni_glass_brain

[KirstieJane]

At the moment we have a lower boundary option in the mni_glass_brain.py code to control the range of the colourmaps, however it doesn't work as there is a bug/decision in the nilearn plotting function to fix the limit to either -vmax or 0. (See issue 1149 at nilearn).

Once this issue is resolved (if it is) then checking to see if our code actually works is important! 😀