As we discussed in class, using fourth-order Runge-Kutta to solve
F'(x)=g(x) is equivalent to integrating 
with
Simpson's rule. Debug your ODE programs using the program ``rk4debug.c''
provided in Appendix A to compare the results of
Runge-Kutta and Simpson's rule for this integration. Be sure to place
your ``derivs_int'' at the top of the file in the indicated location.