use LabelTensor, fix minor, docs

examples/problems/poisson.py

@@ -13,13 +13,14 @@ class Poisson(SpatialProblem):
     domain = Span({'x': [0, 1], 'y': [0, 1]})
 
     def laplace_equation(input_, output_):
-        force_term = (torch.sin(input_['x']*torch.pi) *
-                      torch.sin(input_['y']*torch.pi))
-        return nabla(output_['u'], input_).flatten() - force_term
+        force_term = (torch.sin(input_.extract(['x'])*torch.pi) *
+                      torch.sin(input_.extract(['y'])*torch.pi))
+        nabla_u = nabla(output_.extract(['u']), input_)
+        return nabla_u - force_term
 
     def nil_dirichlet(input_, output_):
         value = 0.0
-        return output_['u'] - value
+        return output_.extract(['u']) - value
 
     conditions = {
         'gamma1': Condition(Span({'x': [-1, 1], 'y':  1}), nil_dirichlet),