[cleanup] mark method SQR as deprecated

dune/common/misc.hh

@@ -61,6 +61,9 @@ namespace Dune {
    *#include <dune/common/misc.hh>
    * \endcode
    */
+  template<class T>
+  T SQR (T t) DUNE_DEPRECATED_MSG("Use method sqr from math.hh instead");
+
   template<class T>
   T SQR (T t)
   {