materials: Review memory management

Currently, we have

QCMesh load_mesh(const char *filename) {
  const auto input = default_userinput(filename);

  load_input(input);
  legacy::load_mpi_globals();

  auto mesh = QCMesh{};

  const auto materials =
      std::vector<Material<DIM> *>{new ExtendedFinnisSinclairCopper<DIM>};

in src/test/testutils/mesh_impl.h

This is for almost sure a memory leak.

Idea:

Change std::vector<Material*> to std::vector<Material> and move the remaining virtual function(s) together with all material specific variables to virtual subclasses of ClusterModel. Material will get a std::unique_ptr<ClusterModel> as a member. This way each material will manage its memory by itself.