Refactor QuasistaticSimulator (again....).

[pangtao22]

Like MBP, QuasistaitcSimulator (which perhaps should be renamed to QuasistaticPlant?) should be context-less. It should only contain constant methods that modify contexts (states) passed in as function arguments.

In contrast, the QuasistaticDynamics class, which has been used a lot in the planning_through_contact repo, should keep track of contexts and update them with calls to methods of QuasistaticPlant.

In addition, all the methods that convert between state dictionaries and state vectors should belong to QuasistaticPlant. Visualization code, on the other hand, should belong to QuasistaticDynamics.

[pangtao22]

Likewise, QuasistaticSimParams should be separated into

• parameters that define the environment, such as n_d and gravity, which should be part of QuasistaticPlant, and
• parameters such as ForwardMode, BackwardMode and h should belong to the params of QuasistaticDynamics, or an equivalent class with a different name.