Put a tolerance around acos to avoid small numerical errors leading to Nans when the value creeps just over 1.0 or less than -1.
Put a tolerance around acos to avoid small numerical errors leading to
Nans when the value creeps just over 1.0 or less than -1.


git-svn-id: 4705079bc6b8aadf675e3696f5c015a6aa4916e3@1062 3c1deb5b-d424-0410-962d-aba41a686d42

hsorby/zinc,hsorby/zinc,hsorby/zinc,OpenCMISS/zinc,OpenCMISS/zinc,OpenCMISS/zinc,hsorby/zinc,OpenCMISS/zinc