simplify index generation in DynamicSpatialOrganizer => speedups

thanks Gerardo for pointing that out

core/src/dynamic_spatial_organizer.hh

@@ -286,9 +286,8 @@ private:
   mutable WithinList result_buffer_;
 
   Index gen_index(const geom::Vec3& pos) const {
-    Index nrvo(static_cast<int>(std::floor(pos[0]/delta_ + Real(0.5))),
-               static_cast<int>(std::floor(pos[1]/delta_ + Real(0.5))),
-               static_cast<int>(std::floor(pos[2]/delta_ + Real(0.5))));
+    Index nrvo(std::floor(pos[0]/delta_), std::floor(pos[1]/delta_),
+               std::floor(pos[2]/delta_));
     return nrvo;
   }
 };