Add a vtkMath method to compute the angle between two vectors

Use the atan2 method rather than the seemingly simpler acos(dot) method for
better precision near 0. This method should be robust for vector inputs of
any magnitude, including 0.

Thanks to David Gobbi for the inspiring discussion on the mailing list:
http://vtk.1045678.n5.nabble.com/Angle-between-two-vectors-td5725202.html

Change-Id: I712ca46a83a12e8e424ab05521176ff545c67610
Merge topic 'add-angle-between-vectors-method' into master

80498e4c Add a vtkMath method to compute the angle between two vectors