Feature request: command options to control image focus

[maxpietsch]

• -voxel: allow 3D crosshair position to be set in voxel units
• -scanner: allow 3D crosshair position to be set in scanner units
• -volume set the volume index
• -index axis value: set the index of an arbitrary axis (for instance volume group, a feature also missing in mrview)

Requires support for floating-point position indices and sanity checks for non-spatial positions being integer-valued.