Virtual points tracking for different points in CT space

[Stephen Gatesy]

We need to use a virtual point on the bird's pelvis to serve as a marker for a virtual point on the bird's femur. But because we disarticulate the skeleton before scanning, these two points have different coordinates in CT space. So somehow we need to have a point on one rigid body act as a marker for a second rigid body as with the existing virtual point workflow, but each needs to have its own CT centroid.

[Benjamin Knorlein]

Quick fix : Should be < 45 min (adjusting also saving)

[Benjamin Knorlein]

fixes #2 : Virtual points can be defined by 2 CT point coordinates and a rigid body index

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