support cluster extent

[nicoalee]

Currently we only have cluster size in cubic millimeters in the study edit extraction phase UI. After a conversation with a user regarding our platform, they mentioned that having a cluster extent column would also be a useful feature.

[jdkent]

so the relevant columns for a point could be: cluster extent, cluster measure, voxel_size (voxel size is only relevant if the cluster extent is measured in voxels and not in mm^3 aleady.