Added median methods for Vector3 and Quaternion

Added `Quaternion.median(Quaternion[])` and `Vector3.median(Vector3[])` methods to calculate the geometric median. It is robust against outliers and therefore is essential when dealing with real world data, e.g real world rotations and translations. I implemented them in the first place for my augmented reality application where we had calculated object transforms coming from the device camera. The full changelog is the following:

- 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