Draft: Code that tests various hyperelastic energy functionals for quasiconvexity

This branch is for Patrizio Neff's hunt for an energy that is rank-1 convex but not quasiconvex.

The code in this branch has been used in the paper

Voss, Martin, Sander, Kumar, Kochmann, Neff: "Numerical Approaches for Investigating Quasiconvexity in the Context of Morrey’s Conjecture" DOI: https://doi.org/10.1007/s00332-022-09820-x

I am not sure whether this should ever get merged. Maybe it is too special. But I want it to be a merge request for some extra visibility.