[utility] Add PriorityTag class

This allows to give priorities to otherwise ambiguous
overloads. It's main use is to make multiple overloads
work, when only some are ruled out via SFINAE. This is
e.g. used in the concepts check that I'll move to
dune-common soon.

The implemention is trivial but makes such applications
more readable and idiomatic compared to other approaches
where, e.g., null pointers are passed to overloads
have T* and void* dummy arguments.

Due to it's simplicity a separate header may not be justified,
but none of the existing seems to fit.

dune/common/prioritytag.hh

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifndef DUNE_COMMON_PRIORITYTAG_HH
+#define DUNE_COMMON_PRIORITYTAG_HH
+
+#include <cstddef>
+
+namespace Dune {
+
+  /**
+   * @addtogroup Common
+   *
+   * @{
+   */
+
+  /**
+   * \brief Helper class for tagging priorities.
+   *
+   * When using multiple overloads of a function
+   * where some are removed from the overload set
+   * via SFINAE, the remaining overloads may be ambiguous.
+   * A prototypic example would be a default overload
+   * that should be used if the others do not apply.
+   *
+   * By adding additional arguments of type PriorityTag<k>
+   * with increasing priority k to all overloads and calling
+   * the method with PriorityTag<m> where m is larger or equal
+   * to the maximal used priority, those can be made unambiguous.
+   *
+   * In this case the matching overload with highest priority
+   * will be used. This is achieved by the fact that PriorityTag<k>
+   * derives from all types PriorityTag<i> with i les than k.
+   *
+   * \tparam priority The priority of this tag.
+   */
+  template<std::size_t priority>
+  struct PriorityTag : public PriorityTag<priority-1>
+  {};
+
+  /**
+   * \brief Helper class for tagging priorities.
+   *
+   * PriorityTag<0> does not derive from any
+   * other PriorityTag.
+   */
+  template<>
+  struct PriorityTag<0>
+  {};
+
+  /**
+   * @}
+   */
+}
+
+#endif // DUNE_COMMON_PRIORITYTAG_HH