make torus parametrization a bit smoother

Before the torus was constructed by
* evaluating the vertices on the torus
* interpolating the unit nodes to each flat element
* ballooning up the nodes to lie on the torus again

Now, it is constructed by
* getting the vertices and nodes on the flat (u, v) grid
* evaluating the vertices and nodes on the torus at the points from
  the flat grid

This results in slightly curvier looking elements.