Center of Mass returns NaNs about k

[AndrewJSchoen]

Digging in a bit further, it seems that this is a problem with the URDF reader urdf-rs. Using the example urdf in this repository produces the following output when I print the robot's first link, which contains inertial values:

Link { 
    name: "root_body", 
    inertial: Inertial { 
        origin: Pose { 
            xyz: [0.0, 0.0, 0.0], 
            rpy: [0.0, 0.0, 0.0] 
        }, 
        mass: Mass { 
            value: 0.0 
        }, 
        inertia: Inertia { 
            ixx: 0.0, ixy: 0.0, ixz: 0.0, iyy: 0.0, iyz: 0.0, izz: 0.0 
        } 
    },
    ...
}

Since those are always zero'd out, it results in a NaN when the total mass is divided.

[taiki-e]

Fixed in openrr/urdf-rs#57.