Switch the poisson_dg_3d input to symbolic

test/sumfact/poisson/poisson_dg_3d.ufl

 cell = "hexahedron"
 
-#x = SpatialCoordinate(cell)
-#c = (0.5-x[0])**2 + (0.5-x[1])**2 + (0.5-x[2])**2
-#g = exp(-1.*c)
-#f = 6*(1.-c)*g
-
-f = Expression("Dune::FieldVector<double,2> c(0.5); c-= x; return 4.*(1.-c.two_norm2())*std::exp(-1.*c.two_norm2());", cell=cell)
-g = Expression("Dune::FieldVector<double,2> c(0.5); c-= x; return std::exp(-1.*c.two_norm2());", on_intersection=True, cell=cell)
+x = SpatialCoordinate(cell)
+c = (0.5-x[0])**2 + (0.5-x[1])**2 + (0.5-x[2])**2
+g = exp(-1.*c)
+f = 6*(1.-c)*g
 
 V = FiniteElement("DG", cell, degree)