program to solve the Lane-Emden equation numerically the equation to solve is
d^2x/dt^2 = - ((2/t)*(dx/dt) + x**n)
where x is the dimensionless density function (psi) and t is the dimensionless length (xi)
initial conditions are x = 1 and dx/dt = 0 and the power index is n.
analytical solutions are known for n=0, 1, 5.