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.
This change isn't right because this is not a Component. It is an internal SimTK::System used to integrate a Bezier curve, I believe.