Type change for efficiency in CellType::create_entities.
New implelentation in TopologyComputation::compute_entities.
Update ChangeLog.
Undo temp fix for CGAL/clang problem.

Proper fix is in another branch.
Add check on Boost version when using reserve for unordered_map.

boost::unordered_map::reserve was introduces in Boost 1.50.
Merge branch 'garth/new-topology-computation'