BUG: energy_data plot not working for 3 dof sims

[aZira371]

Describe the bug

Currently if a 3 dof flight objects calls flight.plots.energy_data(), it gives a net_thrust not found error.

To Reproduce

Create a 3 dof sim using point mass motor and point mass rocket. Run a 3 dof flight sim on them. Try to plot flight.plots.energy_data(). This should give an error on missing net_thrust.

Expected behavior

energy_data plot give a valid graph output

[Gui-FernandesBR]

@aZira371 when reporting bugs like that, try to add the traceback as well so you have a more powerful prompt later.