2-D Grid and Interpolation Parameters

This thorn solves equation (D13.12) on a 2-D $(\eta,\theta)$ grid. However, Cactus needs a 3-D grid, typically with Cartesian coordinates. Therefore, this thorn interpolates $\psi$ and its $(\eta,\theta)$ derivatives to the Cartesian grid. More precisely, for each Cactus grid point, this thorn calculates the corresponding $(\eta,\theta)$ coordinates, and interpolates the 2-D solution to that point.