Include_in_solver flag for external subsystems in mission.

[Kenneth-T-Moore]

Desired capability or behavior.

1. Move the throttle balance solver loop in missionODE into its own subsystem.
2. Allow any external subsystem builder to specify whether it should be added into the top system, or into the solver subsystem. (include_in_solver is a placeholder name)
3. Make sure auto-ordering is turned on for the mission ode and the solver subsystem.

Is your feature request related to a problem? Please describe.

The motivation for this the need for the OAS subsystem to not be under a solver. This is feasible because the drag does not depend on the solver independent: throttle.

Associated Bug Report

No response