The ExternalSourceBesselOutput block allows to output analytically calculated fields at a
particular point in space. The columns denote the x, y, and z coordinate of the point.
Please be aware that ExternalSource lives on the grid of the system that it is applied to.
Therefore, it might not be evaluated at every point in space. When comparing, please be sure
to check the log and compare if your required point in space matches the evaluated position.