init_from_radians
Description:
Initializes a Quaternion using the values of the Euler angles on each axis.
See also: init_from_euler
Parameters:
| this | |
| rad_x | rotation angle on the X axis (yaw), in radians | 
| rad_y | rotation angle on the Y axis (pitch), in radians | 
| rad_z | rotation angle on the Z axis (roll), in radians | 
Returns:
| the initialized quaternion |