Make sure the testPair passes. Old bug.

tests/testPair.c

@@ -276,6 +276,9 @@ int main(int argc, char *argv[]) {
   }
 
   space.periodic = 0;
+  space.dim[0] = 3.;
+  space.dim[1] = 3.;
+  space.dim[2] = 3.;
 
   engine.s = &space;
   engine.time = 0.1f;