Add vehicle efficiency metrics

Desired capability or behavior.

Calculate energy consumption per available seat-mile and per per tonne-kilometer as an output of the Performance subsystem

Suggested implementation.

In performance post-mission, add a component that calculates the metrics:
energy/seat-mile (MJ/pax * mi) = total mission energy consumption / (number of passengers * mission range)
energy/tonne-km (MJ/t * km) = total mission energy consumption / (useful load * mission range)

I am suggesting useful load be used rather than passenger + cargo mass as useful load is more generic and broadly applicable to future aircraft concepts that might have unusual configurations, but a simple sum of pax + cargo might be more intuitive for users. Units are given in openmdao format (t is metric ton)

Start by locating the Performance subsystem's post-mission entry point and the existing outputs for total mission energy, passengers, useful load, and mission range. Add the two requested efficiency metrics using the stated units and formulas, resolve the useful-load interpretation, and verify the outputs with the subsystem's existing tests.