Test case input for y''' = 0 with solution 1.

Run like './step3 -0 0 -1 0 -2 0'
Test case input for y''' = 0 with solution 1+x

Run like ./step3 -0 0 -1 0 -2 0
Test case input for y''' = 0 with quadratic solution

Solution is (2 + 2*x + x*x)/2.

Run like ./step3 -0 0 -1 0 -2 0 -t 0.001.

Notice the solution requires smaller than the default step size
to avoid appreciable discretization error.
Reset to zero-zero-zero initial conditions

Testing in prior commits does not verify the advance() logic
for more than a leading y'''=0 equation, but it does suggest
that I've made no glaring mistakes that I won't be able
to shake out later.