Fix the polynomial degree the ABF element reports about itself.

As stated in the original paper (http://www-users.math.umn.edu/~arnold/papers/vecquad.pdf),
section 5, and as also stated in our discussion of the PolynomialsABF element, the
ABF space of order 'r' actually contains polynomials of degree 'r+2'. Report this
accurately.

Without this, the computation of embedding matrices fails in 3d because we integrate
the least squares matrix terms with a quadrature formula of too low order.

source/fe/fe_abf.cc

@@ -45,7 +45,9 @@ FE_ABF<dim>::FE_ABF (const unsigned int deg)
   FE_PolyTensor<PolynomialsABF<dim>, dim> (
     deg,
     FiniteElementData<dim>(get_dpo_vector(deg),
-                           dim, deg+1, FiniteElementData<dim>::Hdiv),
+                           dim,
+                           deg+2,
+                           FiniteElementData<dim>::Hdiv),
     std::vector<bool>(PolynomialsABF<dim>::compute_n_pols(deg), true),
     std::vector<ComponentMask>(PolynomialsABF<dim>::compute_n_pols(deg),
                                std::vector<bool>(dim,true))),