added some more examples of ufl models with explicit/implicit parts

pydemo/CMakeLists.txt

 dune_add_subdirs(euler)
+
+set(uflfiles
+  advection.ufl
+  diffusion.ufl
+  advectiondiffusion.ufl
+)
+
+dune_fem_add_elliptic_models(${uflfiles})
+
+set(headers)
+foreach(uflfile ${uflfiles})
+  get_filename_component(base ${uflfile} NAME_WE)
+  list(APPEND headers ${base}.hh)
+endforeach()
+
+add_custom_target(models DEPENDS ${headers})

pydemo/advection.ufl

+space = Space(2, 1)
+u, v = TrialFunction(space), TestFunction(space)
+x = SpatialCoordinate(space.cell())
+
+b = as_vector( [1,1] )
+models = {"Explicit":u[0]*dot(b,grad(v[0])) * dx}

pydemo/advectiondiffusion.ufl

+space = Space(2, 1)
+u, v = TrialFunction(space), TestFunction(space)
+x = SpatialCoordinate(space.cell())
+
+b  = as_vector( [1,1] )
+Fc = u[0]*dot(b,grad(v[0])) * dx
+Fd = inner(grad(u),grad(v)) * dx
+
+g1 = as_vector([x[1]*cos(x[0]),sin(x[0])])
+bc1 = DirichletBC(space, g1, 1)
+
+models = {"Explicit":Fc, "Implicit":[Fd,bc1]}
+

pydemo/diffusion.ufl

+space = Space(2, 1)
+u, v = TrialFunction(space), TestFunction(space)
+x = SpatialCoordinate(space.cell())
+
+F = inner(grad(u),grad(v)) * dx
+
+g1 = as_vector([x[1]*cos(x[0]),sin(x[0])])
+bc1 = DirichletBC(space, g1, 1)
+
+models = {"Implicit":[F,bc1]}