add a euler module

pydemo/euler/euler.py

+from ufl import *
+exec(open("euler.ufl").read())
+
+def RiemanProblem(UL,UR):
+    x = SpatialCoordinate(space.cell())
+    return conditional(x[0]<0,as_vector(UL),as_vector(UR))
+def Sod():
+    return RiemanProblem( Model.toCons([1,0,0,1]),
+                          Model.toCons([0.125,0,0,0.1]) )