fix scaling issue for 1D advection (#46)

Author: kmario23

pdebench/data_gen/data_gen_NLE/AdvectionEq/advection_multi_solution_Hydra.py

@@ -261,7 +261,7 @@ def flux(u):
         fL = uL * beta
         fR = uR * beta
         # upwind advection scheme
-        f_upwd = (fR[1:cfg.multi.nx+2] + fL[2:cfg.multi.nx+3]
+        f_upwd = 0.5 * (fR[1:cfg.multi.nx+2] + fL[2:cfg.multi.nx+3]
                - jnp.abs(beta)*(uL[2:cfg.multi.nx+3] - uR[1:cfg.multi.nx+2]))
         return f_upwd