gtPlaceSubVolume: added yet another possible behavior

It's now possible to both assign the single values of the volume, or just an index per each voxels

Passing an index == 0 will select the "copy volume values" behavior

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

git-svn-id: https://svn.code.sf.net/p/dct/code/trunk@322 4c865b51-4357-4376-afb4-474e03ccb993