Connectivity for Agglomeration

[fdrmrc]

In #18, a data structure to store info about the neighboring cells and faces of an agglomeration has been constructed. It allows on to associate to faces of an agglomeration information about the neighboring entities.

In particular, given a master cell cell of an agglomeration, and an agglomerated face agglo_f, we can associate to the pair
(cell,agglo_f) a tuple containing:

• the local face index seen from the agglomeration
• a cell_iterator to the neighboring cell (which is outside of the agglomeration)
• the local face index seen from outside

This should be enough to assemble a matrix, I plan to add what is really necessary tomorrow/next days. For the moment, one can do:

   // assume agglomeration has been set up with an AgglomerationHandler ah
  // DoFHandler    agglo_dh
  for (const auto &cell :
       agglo_dh.active_cell_iterators() |
         IteratorFilters::ActiveFEIndexEqualTo(AggloIndex::master))
    {
      unsigned int n_agglomerated_faces_per_cell =
        ah.n_agglomerated_faces_per_agglomeration(cell);

      for (unsigned int f = 0; f < n_agglomerated_faces_per_cell; ++f)
        {
          const auto& infos = ah.master_neighbors[{cell, f}]; //get information
          const auto &[local_face_idx, neighboring_cell, local_face_idx_out] = info_neighbors;
            }
        }
    }