[doc] Add doxygen group for type utilities

This is intended to contain everything related to type
computations, like, e.g., type traits, helpers for
overload resolution, ...

doc/doxygen/modules.txt

@@ -7,3 +7,10 @@
   @brief Implementations of the STL allocator concept
   @ingroup Common
 */
+
+
+/**
+  @defgroup TypeUtilities Type Utilities
+  @brief Type traits, overload helpers, and other utilities for type computations
+  @ingroup Common
+*/

dune/common/prioritytag.hh

@@ -8,7 +8,7 @@
 namespace Dune {
 
   /**
-   * @addtogroup Common
+   * \ingroup TypeUtilities
    *
    * @{
    */