[discretization] add fem as discretization method

doc/doxygen/modules.txt

@@ -452,6 +452,11 @@
      * \defgroup StaggeredDiscretization Staggered FV scheme
      * \brief A staggered finite volume scheme with degrees of freedom at cell-centers and facets.
      */
+    /*!
+     * \ingroup Discretization
+     * \defgroup FEMDiscretization Finite element method
+     * \brief The finite element method
+     */
 /* ***************** Flux ******************/
 /*!
  * \defgroup Flux Flux

dumux/discretization/method.hh

@@ -35,7 +35,7 @@ namespace Dumux {
      */
     enum class DiscretizationMethod
     {
-        none, box, cctpfa, ccmpfa, staggered
+        none, box, cctpfa, ccmpfa, staggered, fem
     };
 
 } // end namespace Dumux