Skeletonise3D: refactor in preparation for removal (S3D is now in core)
Skeletonise3D: Remove redundant methods

Now that Skeletonize3D is included in ij.plugin.filter, most of this
code is not needed. However, the expected behaviour of creating a
skeletonized copy of the stack is retained. It did require removing a
public class so any API change blame should point here.
ImageCheck: require IJ 1.46j or later due to inclusion of Skeletonize3D
New plugin to calculate triple point angles
Add usage reporting string
Trig: add method to measure 3D angles
SkeletonAngles: allow calculation of angle at any point along edge
Centroid: calculate the centroid of skeleton analysis Point list
SkeletonAngles: move centroid calculation to Centroid class
SkeletonAngle: add some debug output
Trig: fix bug in angle calculation
Trig: add javadoc for angle3D()
Trig: add method to measure 3D angles
Trig: fix bug in angle calculation
Trig: add javadoc for angle3D()
Centroid: calculate the centroid of skeleton analysis Point list
CentroidTest: add test for ArrayList<Point> override method
CentroidTest: remove unused import
TrigTest: add test for angle3D() method
SkeletonAngles: refactor for testing and to expose public API