Special case handling: center or the ellipse

A special case we will disgard in the algorithm is the ellipse center. This point is easily detected in a preliminary check using a test like $\sqrt{r^2+z^2}<\varepsilon_0$. The ellipse points closest to the center are both endpoints of the minor axis, we arbitrarily select the point having $\varphi>0$: $\varphi=+\frac{\pi}{2}$, $h=-(1-f)a_e$.