Remove boost/thread from rtree, include header for hash_combine in unit test.

The `static_rtree.hpp` header included `<booost/thread.hpp>` without using
anything from this header.

Removing it showed why:

the unit test for the rtree no longer built, since it was missing symbols
for Boost's `hash_combine`, used in the unit test.

Instead of relying on `<boost/thread.hpp>` including the proper header
for `hash_combine` by chance that we only use in the unit test, do the
following:

- remove `<boost/thread.hpp>` from the rtree implementation
- add `<boost/functional/hash.hpp>` to the rtree unit test

As always, include what you use.

data_structures/static_rtree.hpp

@@ -50,7 +50,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <boost/assert.hpp>
 #include <boost/filesystem.hpp>
 #include <boost/filesystem/fstream.hpp>
-#include <boost/thread.hpp>
+#include <boost/range/irange.hpp>
 
 #include <tbb/parallel_for.h>
 #include <tbb/parallel_sort.h>

unit_tests/data_structures/static_rtree.cpp

@@ -32,6 +32,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "../../util/floating_point.hpp"
 #include "../../typedefs.h"
 
+#include <boost/functional/hash.hpp>
 #include <boost/test/unit_test.hpp>
 #include <boost/test/test_case_template.hpp>
 #include <boost/mpl/list.hpp>