[doc][doxygen] introduce common group

doc/doxygen/Doxyfile.in

@@ -790,7 +790,11 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = @CMAKE_CURRENT_SOURCE_DIR@/frackit/ @CMAKE_CURRENT_SOURCE_DIR@/test
+INPUT                  = @CMAKE_CURRENT_SOURCE_DIR@/doc/doxygen/mainpage.txt  \
+                         @CMAKE_CURRENT_SOURCE_DIR@/doc/doxygen/modules.txt  \
+                         @CMAKE_CURRENT_SOURCE_DIR@/frackit/  \
+                         @CMAKE_CURRENT_SOURCE_DIR@/test/
+
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses

doc/doxygen/modules.txt

+/*!
+ * \file
+ * \brief This file contains all module definitions for the doxygen documentation.
+ */
+
+/* ***************** Porousmediumflow ******************/
+/*!
+ * \defgroup Common Common
+ * \brief Common classes, functions, properties and concepts
+ */