Artificial viscosity model:

Solve u_t + F(u)_x= (eps u_x)_x where eps u_xx is an artificial 
viscosity term. 

How to choose eps ???

Look at the steady convection - diffusion problem

eps u_xx + V u_x = 0   eps: viscosity term
                       V  : convective speed 

If we approximate this with a central 2nd order scheme we know that

eps >= V DX /2   (1)

to have a solution free from oscillations.

In the baraflux.m we use the following viscosity model:

eps(j)= DX*(abs(V) + c)*(C2*sw(j) + C0)

C2 is a constant, chosen in the order of 1/2, compare (1) 

|V| + c is the velocity scaling (in order to get the right dimension
        & size). The velocity scaling is chosen as the characteristic
	speed where c is the speed of sound, also called Sspeed in 
	the program and V is the speed of the gas.  

sw(j) is a switch in order to locate the
shock. 

sw(j)=|rho(j+1) -2rho(j) + rho(j-1)|/(rho(j+1)+2rho(j)+rho(j-1)) 

C0 is a "background" diffusion constant