Added, for testing and comparison purposes, experimental "nest" container under "experimental" folder, a C++03 backport and adaptation to use Boost.Container internals of Joaquín López Muñoz's "hub" container proposal (https://lists.boost.org/archives/list/boost@lists.boost.org/message/KS67XSLDY64Q37BDFUAEUXHTQSUL6HC7/).

Author: igaztanaga