WIP 6DOF trajectory simulator for rocket landing - for testing propulsive landing control system "Horatio".

Next changes:

• Actuator models for gridfins and engine (both gimble and throttle)
• Propellant slosh in tanks
• More accurate grid fin aerodynamics
• More accurate rocket body aerodynamic data
• Better handling of flight phases (engine not lit, lighting, lit)
• Add ascent phase as well to simulate entire flight trajectory