message: Improve ghost faces documentation.
diff: @@ -153,14 +153,26 @@ protected:
  are used, one should read the integrality of the mesh.cpp, pmesh.cpp,
  ncmesh.cpp, and pncmesh.cpp files and reverse engineer the code.
  What I found about them so far:
  - - Their number is faces_info.Size() - GetNumFaces()
  - - Most of them only countains -1 for all values (that seems to be
  - explained by pncmesh.cpp:1073-1074), the technical documentation below
  - wonders if these are master non-conforming faces, I think they're
  - probably just ghost ghost cells, unused allocated memory space.
  + - Their number is faces_info.Size() - GetNumFaces(), this seems to
  + include a lot of irrelevant uninitialized ghost faces.
  + - Most of the ghost faces only countains -1 in all their attributes
  + (that seems to be explained by pncmesh.cpp:1073-1074).
  + The technical documentation below wonders if these are master
  + non-conforming faces, I think they're probably just ghost ghost cells,
  + unused allocated memory space.
  However, I still tag them as MasterNonConforming in GetFaceInformation
  - They seem to be used as a convenience layer only in NCMesh, why not
  using them all the time?
  + - Due to clashing conventions:
  + 1. On shared faces elem1 is always the local element
  + 2. On non-conforming faces elem1 is always the slave element
  + The case of shared non-conforming faces is incompatible with the local
  + face being also the master face. For this reason ghost faces only
  + assume elem1 to be the local face.
  + - It seems that ghost faces are only used for shared non-conforming faces
  + where elem1 is master and local. I think non-conforming faces where the
  + local face is slave are treated through conforming shared faces. (This
  + hack has to be confirmed)
  */
  struct FaceInfo
  {
  