Triple contraction for 3rd-order tensors (Hessian of displacement) in Gridap - "This function is not yet implemented?"

[Tony-2629]

Hi Gridap.jl community!

I am working on a gradient-enhanced hyperelastic (regularising the Hessian of displacement, i.e. $\nabla(\nabla u)$, and I am stuck on triple contraction for 3rd-order tensors.

Problem
I want to compute term like:

$$\int_{\Omega} \nabla^{2}(u) \odot \nabla^{2}(v) \quad \int_{\Omega} \nabla^{2}(u) : \nabla^{2}(v)$$

where ∇²v and ∇²u are 3rd-order tensors (shape (2,2,2) in 2D).

[JordiManyer]

Hey @Tony-2629 ! As it says in the error, this function is not yet implemented. You can have a look at this file and try to implement the missing signatures. Feel free to open a PR with the changes, I will gladly accept it.