Calculate relativistic and spin-orbit fine structure corrections
E_fs = E_0 * (alpha^2 * Z^2 / n) * (n/(j+1/2) - 3/4)
alpha = e^2/(4*pi*epsilon_0*hbar*c) = 1/137.036
E_0 = -13.6 * Z^2 / n^2 eV
