Workflow for trajectory placement on post-CT

[d-vogel]

Annotation of electrode position on post-CT is a typical task, it can be made easier with the probeview module, but the process is for now not exactly super smooth.
This requires:

• registering the post-CT with the pre-CT using the linear registration tool from slicer
• Then the original post-ct should be used as output image for export in stereo points, and the Lesksell2RAS and Pre-CT2post-CT should be assembled in the right order.
• extra bonus points: allow exporting directly as ea_reconstruction.mat for LeadDBS