Curvature tensor and projections

Hi, I'm trying to calculate a curvature tensor on a 3d surface mesh. My dynamic equations involve $C_ij$ and the trace of the curvature tensor $C^k_k$.

Gridap seems happy for me to do the following :

```
model = GmshDiscreteModel("icosphere.msh")

Γ = Triangulation(model)

order = 2
Vn = FESpace(
Γ,
ReferenceFE(lagrangian, VectorValue{3,Float64}, order),
conformity=:H1
)

Vcij = FESpace(
Γ,
ReferenceFE(lagrangian, TensorValue{3,3,Float64}, order),
conformity=:L2
)

Vckk = FESpace(
Γ,
ReferenceFE(lagrangian, Float64, order),
conformity=:L2
)

n_Γ = get_normal_vector(Γ)

I3 = one(TensorValue{3,3,Float64})
P = I3 - n_Γ ⊗ n_Γ

Cij = - (P ⊙ ∇(n_Γ)) ⊙ P
Ckk = tr(Cij)

Cij_interp = interpolate(Cij, Vcij)
Ckk_interp = interpolate(Ckk, Vckk)

normals = interpolate(n_Γ, Vn)
```

When I save, I get the expected normals, but the other quantities are all zero. Is this not allowed? Is there some other way I could define these? The Gridap machinery seems well-suited to this sort of thing, but perhaps I'm missing something.

Many thanks!

Start by reproducing the supplied Gridap.jl example with GmshDiscreteModel, Triangulation, get_normal_vector, and the three interpolate calls. Compare the saved Cij and Ckk fields with the normals, then determine whether these projected curvature expressions are supported as written or require a documented alternative.

Written by the indexing model from the issue text.