ENH: update MultiVolumeExplorer module

* BUG: fix 3058 - support relative change plotting
* BUG: fix static plotting
* STYLE: clean up the code a bit


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22380 3bd1e089-480b-0410-8dfb-8563597acbee