Enable sensitivity analysis

[m-bossart]

My goal is to make the building of a simulation and the solution differentiable. This will enable sensitivity analysis of the output of a dynamic simulation wrt some parameters of the system model. The first commit addresses the first step which is to build out the capability of passing and handling parameters as a flat vector (similar to the states).

• [x] Pass parameters as a flat vector when constructing the differential equation problem.
• [x] Simulation flow upgrades
• [x] Changes for compatibility with automatic differentiation (Zygote)
• [x] Sensitivity analysis API