Extend component realize methods and addreses #184

Can now invoke realization to computational stage through Component's public (non-virtual) interface.
Virtual realize methods renamed to `extendRealize<...>()`.
Realization occurs with the System the component was added to.  If system reference is empty, you also now get a (hopefully) meaningful message.
Better the way it was: `system.realize(s, Stage::Acceleration);`.