Add handling of flat boundaries made of faces

src/mesh/CMakeLists.txt

@@ -18,6 +18,7 @@ add_library(
   MeshBuilderBase.cpp
   MeshDataManager.cpp
   MeshFaceBoundary.cpp
+  MeshFlatFaceBoundary.cpp
   MeshFlatNodeBoundary.cpp
   MeshLineNodeBoundary.cpp
   MeshNodeBoundary.cpp