Issue with Tutorial 03

1.  The specific implementation of the `Legendre` method within `MultiVariatePoly` is not provided.
2.  Is $u(x, 0) = u_0(x) = - \sin(\pi x)$ supposed to be $u(0, x) = u_0(x) = - \sin(\pi x)$?
3.  The `...domain (t,x) in [0,0.1]x[-1,1]` should be `[0, 0.6]`.
4.  The code snippet `grad_u_sq_x = torch.autograd.grad(u_sq.sum(), input_int, create_graph=True)[0][:,1]` was not completely removed.