Allow full tet ruleset

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 (closed)

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.

mentioned in issue core/dune-grid#27 (closed)

mentioned in commit 49651efe

merged