Function klein_gordon_kernel 

pub fn klein_gordon_kernel<R>(
    psi_manifold: &SimplicialManifold<R, R>,
    mass: R,
) -> Result<CausalTensor<R>, PhysicsError>
where
    R: RealField + MaybeParallel + FromPrimitive + Default + PartialEq + Debug,

Calculates the Klein-Gordon operator action: $(\Delta + m^2)\psi$.