Add part bonus to rotor diameter in thrust calculations

[raycrasher]

Summary

Features "Add part bonus to rotor diameter in thrust calculations"

Purpose of change

This allows creation of new lifting parts that have a smaller danger zone like ducted fans or VTOL thrusters.

Describe the solution

Small change to vehicle::lift_thrust_of_rotorcraft that adds the part bonus to the diameter. This does not affect collision diameter.
So if rotor_diameter = 5 and bonus = 10 then the rotor will have a thrust of a 15-diameter rotor but have a collision diameter of just 5.

Describe alternatives you've considered

Increasing rotor area (and thus the collision zone) is currently the only way to improve lift.

Testing

Game compiles.

Old rotors function as normal.

Replace "small_civilian_rotors" JSON:

"rotor_diameter": 1,
"bonus": 7

The 2seater2 helicopter still flies as normal.
Rotor collision area should be reduced.

Additional context

[Maleclypse]

@Maleclypse tagging myself here to have an email that I can easily pull up for the mad genius to make a personal helicopter design.

[moxian]

I probably should have said something earlier, but now that this is merged, there's little point. Still, I think it's worth raising some concerns (even if those don't change anything). Two concerns, and a nitpick:

• This is adding a code path that is never excercised in game (neither in base game nor in the mainline mods). Why? I don't have a quote handy, but I belive we try pretty hard to only add the code that is used, and this one is not. If this came bundled with a ducted fan vehicle part (and a ducted fan-using vehicle), that would be fair, but right now, it's just code for code's sake?
• How exactly do you calculcate the "additional rotor diameter during lift calculations, in meters"? If a ducted fan has a rotor_diameter of 15 and a bonus of 5 [meters], where do those "5 meters" come from? I doubt it's anywhere on the physical part? If we wanted to have an arbitrary "this part magically generates more lift without increasing the danger area", we should have made the bonus a unitless scaling factor (so "bonus": 0.33 in the 15m/5m example)
• This is more of a nitpick, but reusing the generic "bonus" field for this just feels very bad. It should have a nice readable name like "rotor_effective_diameter_bonus" or (if we got with factor scaling route) "lift_factor"/"lift_bonus_factor".). And if we stick to "extra diameter, in meters", then that "in meters" should really be part of the json, not tucked away into the docs. It should be specified as "bonus": "5m", so that people don't get confused on what exactly that 5 means. But in your defence, rotor_diameter is already not following the best practices here, so this is underestandable.

Are you planning on making followup PRs adding the parts, or did you add it for your personal mod, or some third option?