Poisson's equation solver

Fill in code at the remaining locations indicated ``...'' in ``poisson.c'' to complete your solution of Poisson's equation.

Notes: Be sure to integrate backwards from $r=\infty$ ($u$=1) to $r=0$ ($u$=0): you'll have to change the order of the loop and the values of both $h$ and $dk$! Also, remember the boundary conditions:
$\Phi(\infty)=\Phi(r(u=1))=N_{el}$ and $\Phi'(\infty)=\Phi'(r(u=1))=0$.