BUG: update multivolume modules

Fixed to work with both VTK5 and VTK6 API

https://github.com/fedorov/MultiVolumeExplorer/commit/1053c40846e80c277f20c7af474eaee64ef4a16d
https://github.com/fedorov/MultiVolumeImporter/commit/351226a1dfeb2e9113424f4a42e57390bb17ceb9