Virtual Ruler

A helping function to measure distances in a 3D plot.

This function makes it easier to measure distances in a 3D plot because:
- Selected vertices and distances are kept track of.
- Consecutive measurements are interpreted in three different ways, depending on if and when a measurement is skipped. This allows "snake"-like and "fan"-like distances (see picture) which reduces the number of manual clicks for the user.
- Multiple figures can be used by the function at the same time. So it is possible to make a measurement in figure A and for comparison the same measurement in figure B.