public void setGravity(Vector3f gravity) {
        rigidBody.setGravity(gravity);
        localUp.set(gravity).normalizeLocal().negateLocal();
        updateLocalCoordinateSystem();
    }
Alter the gravity acting on this character. Note that this also realigns
the local coordinate system of the character so that continuous changes
in gravity direction are possible while maintaining a sensible control
over the character.

@param gravity an acceleration vector (not null, unaffected)
jme3-jbullet/src/main/java/com/jme3/bullet/control/BetterCharacterControl.java