[vtkusers] Angle between two vectors

[David Cole]

FYI: I pushed a gerrit topic that adds an AngleBetweenVectors method. 
It uses atan2 in the implementation. Also adds code to the TestMath.cxx 
test to call it with various parameters.

    http://review.source.kitware.com/#/t/3804/

Please review if you're able & interested.


Thanks,
David C.