[ODE] dJointSetAMotorAxis

[Jason Mallios]

Considering I have two bodies attached by ball joint, and I want to add an
AMotor between them in Euler mode using one body's object-space x, y, and
z axes as the AMotor axes, which two axes should I set as AMotor axis 0
and AMotor axis 2 to ensure the automatically-calculated axis equals the
third object-space axis I did not pass it.

Two things have complicated this:

- dJointGetAMotorAxis returns a zero vector for the automatically-
calculated axis in Euler mode, so there is no way to check it.

- dJointGetAMotorAngle only returns current angles in Euler mode.  User
mode always returns the initial angles.  Without current angles, torque
calculations become unnecessarily tricky.

Thanks,
Jason