MeshValueCollection for cell is wrongly handled in parallel

There is a problem in MeshPartitioning::build_mesh_domains when each partition has exactly 1 element and and cells are marked. Then mesh/MeshPartitioning.h:1103 raises a segfault (d - d connectivity is empty). But in general it is wrong anyway, because a marked cell is referenced as the 0 local entity of entities of dimension d, and this is clearly not contained in d - d connectivity (accordingly to dolfin personal definition of d - d connectivity).

An easy fix is:

diff --git a/dolfin/mesh/MeshPartitioning.cpp b/dolfin/mesh/MeshPartitioning.cpp
index bc8853d..3d41634 100644
--- a/dolfin/mesh/MeshPartitioning.cpp
+++ b/dolfin/mesh/MeshPartitioning.cpp
@@ -1099,9 +1099,14 @@ void MeshPartitioning::build_mesh_domains(Mesh& mesh,
       const std::size_t cell_index = it->first.first;
       const std::size_t local_entity_index = it->first.second;

-      const Cell cell(mesh, cell_index);
-      const MeshEntity e(mesh, dim, cell.entities(dim)[local_entity_index]);
-      markers[e.index()] = it->second;
+      if ( dim == D )
+        markers[0] = it->second;
+      else
+      {
+        const Cell cell(mesh, cell_index);
+        const MeshEntity e(mesh, dim, cell.entities(dim)[local_entity_index]);
+        markers[e.index()] = it->second;
+      }
     }
   }
 }

The procedure should be further optimized by properly changing build_mesh_value_collection, because it initializes d - d connectivity, which we know is very expensive and clearly useless in this context.

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