Maneuver structure and schedule

Should provide a schedule of burns in RIC, VNC and inertial frame. These will need start and stop times for the EOMs to be correct.

• Propulsion subsystem
• Maneuver in VNC frame
• Maneuver schedule
• Mass decrement on demand
• GMAT validation of a finite burn without mass decrement: unsure I set this up correctly. Hence the validation will be done against the item below
• Q-Law control for low thrust with mass decrement and validation from paper (and smd) cf. #82
• Prevent obsolete gravity field code from being built, even with unvalidated flag (that part of the code has been unmaintained for a while now).
• Add instantaneous maneuvers in addition to finite burns, cf #84 (closed)