Added support for Euler angles in Quaternion structs

Added support for Euler angles in Quaternion and Quaterniond by adding proper vec3 and pitch-yaw-roll constructors, and also added similar static helper functions. I unit tested them locally and the results match to the calculator found from the bottom of this web page: http://www.euclideanspace.com/maths/geometry/rotations/conversions/eulerToQuaternion/index.htm