Allow to set the TET_RULESET macro

UG contains two implementations of green closures of tetrahedral
elements, and the TET_RULESET macro allows to switch between
the two.  As I recently learned from Peter Bastian, we have been
using the inferior implementation all along, and we should be
enabling TET_RULESET in the future.  As a first step, this patch
allows to set TET_RULESET from the CMake flags.

Incidentally, setting TET_RULESET fixes

  core/dune-grid#27

Beware: when TET_RULESET is set, UG wants to read a table from
the file RefRule.data (in uggrid/lib/ugdata).  This file needs
to be in the current directory, otherwise UGGrid will abort.

config.h.cmake

@@ -22,6 +22,9 @@
 /* Define to 1 if you can safely include both <sys/time.h> and <time.h>. */
 #cmakedefine TIME_WITH_SYS_TIME 1
 
+/* Define to 1 if UGGrid should use the complete set of green refinement rules for tetrahedra */
+#cmakedefine TET_RULESET 1
+
 /* end private section */
 
 /* end dune-uggrid */