ENH: get the basic linear case working

Now the transform is being calculated for the selected landmarks and
is applied to the proper volume node.

Next steps:
- fix the Add button
- make it a live preview of the transform as fiducials are moved
- make sure layout of transforms is working correctly

Other things:
- Add puts you in fiducial place mode?
- other preview options (cross fade, wipe?)
- show transformed fiducials in transformed slice view?