Add stateful SysModel for mujoco Dynamics

[juan-g-bonilla]

Describe your use case
Currently, the MJScene dynamic object cannot have continuous states other than the states of the multi-body. A continuous state is a state whose evolution is given by its time derivative, and thus must be integrated forward in time. spacecraft objects use stateEffector to declare such states (for example, a fuel tank is modeled as a stateEffector whose mass in a continuous state). A similar capability is required for MJScene