Introduce a Simbody-like staging mechanism in OpenSim

[tkuchida]

Stages could include

• model building
  • properties unverified
  • properties finalized
  • connections satisfied
• simulating
• …

and perhaps include Simbody-like SimTK::Stages to resolve issues like #413 and #603.

[aymanhab]

Quoting @aseth1 in #2526

How would you introduce a SimTK::Stage for model building and simulation in OpenSim? How would you leverage it to avoid the explicit calls to the current "staged" methods? How would this be different other than calling Model::realize(Connections)? If that is a more desirable interface, I can see us transitioning to it, but it doesn't change the number of calls or the fact that user's have to know about "connections".

Staging is necessary in Simbody to manage a hidden cache in the State for an entire System. What is the analogous Cache that we need to keep up-to-date? It seems this would be properties themselves. Some properties (like Socket connections) can only be valid after other properties (list of subcomponents) are defined. There is another "staging" order that involves the order in which the system is built and how dependencies are resolved, but that is another issue altogether (#413).