2nd order derivatives fail for high order P elements

[fverdugo]

[The code below requires PR #769]

This fails:

using Gridap

domain = (0,1,0,1)
cells = (2,2)
model = CartesianDiscreteModel(domain,cells) |> simplexify
Ω = Interior(model)
dΩ = Measure(Ω,4)
m = 2
u(x) = VectorValue(sum(x)^m, sum(x)^m)
reffe = ReferenceFE(lagrangian,VectorValue{2,Float64},m)
V = FESpace(Ω,reffe)
uh = interpolate(u,V)
t = Δ(u) - Δ(uh)
@assert sum(∫(t⋅t)dΩ) < 1.0e-12

Independent actions that make the test work:

• removing the call to simplexify
• using m=1
• using u(x) = sum(x)^m and ReferenceFE(lagrangian,Float64,m) (i.e., an scalar valued function)

[fverdugo]

This might be relevant to you @oriolcg