add support for passing std::unique_ptr into SgTEvent::record

The goal is to allow users to do something like this:

auto jets = std::make_unique<xAOD::JetContainer> ();
...
ANA_CHECK (evtStore()->record (std::move (jets), "myjets"));

The old mechanism is still supported, but I'd much rather start teaching people to pass around smart pointers instead of just bare pointers.