[modifiers] pose_noise: separate std_dev for each axis

This makes it possible to specify the noise for each axis separately as
a dict.
If a single value is passed, it is used for all axes.
E.g.:
pose.alter('', 'morse.modifiers.pose_noise.PoseNoiseModifier',
				pos_std={'x':0.01, 'y':0.01, 'z':0.1},
				rot_std=math.radians(1))
or
pose.alter('', 'morse.modifiers.pose_noise.PoseNoiseModifier',
				pos_std={'x':0.01, 'y':0.01, 'z':0.1},
				rot_std={'roll':0.01, 'pitch':0.01, 'yaw':0.05})
[modifiers] imu_noise: dict for separate std_dev on each axis