- improved version of TriaxialStressController, controling not onlt stress but also strain rate
- making the interface more similar (but still not exactly like) PeriTriaxController.
- TriaxialCompressionEngine and ThreeDTriaxialEngine become almost useless
- reflected in the example script which now uses only TriaxialStressController
-micro-fix (no need to define an initial timestep)
-code cleaning
- fix the default value of orientation in utils.box. [1,0,0,0] is NOT a quaternion.