Add stylus visualization

[lukatolo]

To correctly orient the stylus in VTK relative to the head, a manual calibration must be done.
(This is because the head-reference-marker-tool can be placed on the physical head at an arbitrary orientation. Even though the stylus is already calibrated with the camera, it is not calibrated with the physical head, hence the manual step.)

The manual step can be skipped if precise stylus orientation is not important. If the plane of the head-marker-tool is aligned with the physical head's face, the stylus may even look right without calibration.

Additions to code:

• Added class ProbeVisualizer to show stylus in VTK
• StylusPage in task_navigator to do the manual calibration
• Stylus coordinates are corregistered to VTK with def corregistrate_probe
• The stylus can be hidden during navigation with a button in ControlPanel.

Note on aligning passive tracker tools (eg. with Polaris ROM files)
Stylus:

• Tip should point in positive y-axis
• Normal of the passive markers should be in positive z-direction

Head/reference markers:

• The normal of the plane formed by markers should point in positive z direction
• The axis of rotation of the tool that is horizontal relative to the patient's face should be aligned with y-axis

The updated .rom files follow these standards

[lukatolo]

I'll rebase this to the RUFF format today...