Kernel for Poisson's equation

Write a ``derivs_Poisson()'' routine at the location indicated near the top of ``poisson.c''. This routine should implement the first-order version of Poisson's equation as discussed in class,

$$\frac{d}{du}\left(\begin{array}{c}\Phi\\ \Phi'\end{array}\ri... ...}\Phi'\\ -4 \pi n(r) r \end{array}\right) \cdot \frac{dr}{du},$$

where $\phi(r)=\Phi(r)/r$.