Feature: Add dg competence to mesh handle

mesh_handle/README.md

@@ -10,9 +10,8 @@ The folder's most important files are:
 - The [mesh.hxx](mesh.hxx) defines the mesh of the handle. This is the central file of the mesh handle. 
 - The [constructor_wrappers.hxx](constructor_wrappers.hxx) allows to define a mesh handle using a cmesh instead of a forest and provides a very small number of examples where the user needs no cmesh.
 - The [element.hxx](element.hxx) defines the elements (mesh or ghost elements) of the mesh handle.
-- The [competences.hxx](competences.hxx) defines additional competences/functionality of an element to access additional data.
+- In the folder [competences/](competences/README.md), there are files that define additional competences/functionality of an element to access additional data.
 - The [competence_pack.hxx](competence_pack.hxx) is needed to pack element or mesh competences to pass it as template parameter to the mesh.
-- The [data_handler.hxx](data_handler.hxx) provides competences to work with element data.
 - The [mesh_io.hxx](mesh_io.hxx) allows to output results in vtk format.
 
 Headers in the [internal/](internal/) folder are only intended to implement details of the mesh handle, so they should not need to be included for any other purpose.

mesh_handle/competence_pack.hxx

@@ -30,8 +30,9 @@
 
 #pragma once
 
-#include "competences.hxx"
-#include "data_handler.hxx"
+#include "competences/cache_element_competences.hxx"
+#include "competences/element_data_competences.hxx"
+#include "competences/dg_competences.hxx"
 #include "internal/competence_pack_union.hxx"
 namespace t8_mesh_handle
 {
@@ -96,6 +97,9 @@ using empty_mesh_competences = mesh_competence_pack<>;
 template <T8MPISafeType TElementDataType>
 using data_mesh_competences = mesh_competence_pack<element_data_mesh_competence<TElementDataType>::template type>;
 
+/** Predefined mesh competence pack combining all competences that are useful for discontinuous Galerkin methods. */
+using dg_mesh_competences = mesh_competence_pack<remote_ranks_mesh_competence, face_vector_mesh_competence>;
+
 // --- Compute union of competence packs. ---
 /** Compute the unique union of the competences of several competence_pack. This could be
  * \ref t8_mesh_handle::element_competence_pack or \ref t8_mesh_handle::mesh_competence_pack.

mesh_handle/competences/README.md

@@ -0,0 +1,19 @@
+# Competences for the mesh handle #
+
+Definition of additional competences/functionalities that can be used for the t8_mesh_handle::mesh.
+
+- [element_data_competences.hxx](element_data_competences.hxx) provides competences to work with element data.
+- [cache_element_competences.hxx](cache_element_competences.hxx) defines competences to cache functionalities of elements instead of calculating them each time a function is called.
+- [dg_competences.hxx](dg_competences.hxx): competences that are useful for discontinuous Galerkin methods.
+
+### Note for developers or users that want to provide their own competence:
+
+All competences have the same inheritance pattern: 
+We use the CRTP pattern as we may need to access members of the derived classes like t8_mesh_handle::element. 
+The t8_crtp_operator is used for convenience/clear code (avoid to type a static cast explicitly each time we need functionality of TUnderlying).
+Especially for the competences to cache functionality, the access of members is not necessary, such that it is not obvious why we use the crtp. 
+For competences that extend the functionality of the element (see e.g. [dg_competences.hxx](dg_competences.hxx)), this is required. 
+We use it for all competences for consistency and compatibility with t8_mesh_handle::element.
+
+We use t8_crtp_operator instead of t8_crtp_basic to circumvent diamond shaped inheritance pattern in the case that multiple competences are used together. 
+The distinction of the classes is made by the second template parameter of t8_crtp_operator.

mesh_handle/competences.hxx → mesh_handle/competences/cache_element_competences.hxx

@@ -20,7 +20,7 @@ along with t8code; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */
 
-/** \file competences.hxx
+/** \file cache_element_competences.hxx
  * Definition of the additional competences/functionalities that can be used for \ref t8_mesh_handle::mesh.
  * Especially, competences to cache functionalities of elements instead of calculating them each time a function
  * is called are provided.