Added median methods for Vector3 and Quaternion

- Corrected Quaternion.pow() name, was exp() before
- Added Quaternion.log() and Quaternion.exp()
- Added Quaternion.logMap() and Quaternion.expMap()
- Added Quaternion.dist()
- Added Quaternion.median() to calculate geometric median
- Added Vector3.median() to calculate geometric median
Added median method for Matrix4
Added weighted versions of median methods
Refactored Vector median into the Vector interface