Change calculation of coulomb potential

By changing the calculation of the coulomb potential to the one given by
Hans and Jens, the calculations now converge to within 1% of the
reference values of hydrogen and positronium. However, I am unsure if it
is correct, because the previous implemented looked more correct,
although it gave values significantly of the reference (although
converging)
Just cleaning up
Setup for calculating positronium
Add method to calculate orto helium