Fixes dual quaternion skinning not checking if the quaternions are on the same hemisphere. For quaternions $q$ and $-q$ represents the same rotation.

One trivial example is when randomly negate one of the rotations in vQ in tutorial 404, the current implementaion yields unplausible result. It works in tutorial 404 because the input rotations happens to be on the same hemisphere, but this is not guaranteed in general. Similar check is also used in Blender for DQS here.

Checklist

• All changes meet libigl style-guidelines.
• Adds new .cpp file.
• Adds corresponding unit test.
• This is a minor change.