Variable-coefficient traffic flow model.  The flux function is
  f(q,x) = u(x)*q*(1-q)
where the "speed limit" u(x) varies with x. 
It's value in cell i is stored in aux(i,1) (see setaux.f).

Here a Riemann problem is solved with 
   u(x) = 2 for x<0
   u(x) = 1 for x>0

The initial data is set in qinit.f.   
Set ql to 0.13 for Figure 16.9
Set ql to 0.2 for Figure 16.10

The method used is based on splitting the flux difference into f-waves Z
as described in Section ?? and implemented in rp1trvfw.f.

Note that this requires a modified routine step1fw.f in place of the library
routine step1.f in order to implement the modified wave-propagation
algorithm.

The transonic rarefaction case is very sensitive to the flux splitting used.
The approach taken in rp1trvfw.f gives the correct solution for this case
but has not been extensively tested...


WARNING: This example was modified February, 2006 to use
Version 4.3 of clawpack and the associated matlab scripts.