Remove the const from VolumeDataset, and add another const method.
Remove the const from VolumeDataset, and add another const method.

Allows it to be overloaded.  Now returns a mutable or a constant,
depending on what kind of object called it.

git-svn-id: 83c4c97093aeea62ac982fbfd516bf9db27f5c99@884 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok