Implemented checkpoint-restart

[asartori86]

PR mathLab/deal2lkit#322 is required

[luca-heltai]

You should not use this mechanism here. See

https://gitlab.com/code_projects/eigen-laplace/blob/master/source/eigen_problem.cc

and copy that.

[luca-heltai]

In particular:

    std::vector<const PETScWrappers::MPI::Vector *> vectors;
    for (auto &e : eigenfunctions_ghosted)
      vectors.push_back(&e);

    soltrans = SP(new parallel::distributed::SolutionTransfer<dim, PETScWrappers::MPI::Vector>(*dof_handler));
    soltrans->prepare_serialization (vectors);

    triangulation->save ("restart.mesh");
  }

[luca-heltai]

and at resume time:

  eigenfunctions.resize (n_eigenvalues);
  eigenfunctions_ghosted.resize (n_eigenvalues);

  std::vector<PETScWrappers::MPI::Vector *> all_out(n_eigenvalues);
  for (unsigned int i=0; i<n_eigenvalues; ++i)
    {
      eigenfunctions[i].reinit (owned_index_set, MPI_COMM_WORLD);
      all_out[i] = &eigenfunctions[i];
    }

  // Ghosted functions are used internally by soltrans
  if (cycle > 0 && resume_computation == false)
    soltrans->interpolate(all_out);
  else if (resume_computation == true)
    {
      soltrans = SP(new parallel::distributed::SolutionTransfer<dim, PETScWrappers::MPI::Vector>(*dof_handler));
      soltrans->deserialize(all_out);
      resume_computation = false;
    }


  // Now that we have transfered the old ones, reinitialize ghosted vectors.
  for (unsigned int i=0; i<n_eigenvalues; ++i)
    {
      eigenfunctions_ghosted[i].reinit (owned_index_set,
                                        relevant_index_set,
                                        MPI_COMM_WORLD);
      eigenfunctions_ghosted[i] = eigenfunctions[i];
    }