FGTurbine Thrust N1/N2 Selection

[Octal450]

I'm submitting a ...

• feature request

Describe the issue
Turbojets and low bypass turbofans most of the thrust comes from the N2 compressor, as in

jsbsim/src/models/propulsion/FGTurbine.cpp

Line 220 in 9ea4925

thrust = idlethrust + (milthrust * N2norm * N2norm);

But on a high bypass turbofan, most of the thrust comes from the N1 compressor (the fan).

I would like the option to select between the above N2 calc code and N1:
thrust = idlethrust + (milthrust * N1norm * N1norm);

As a further request, I'd like the option to override this entirely and provide my own calculation (to combine N1/N2 components) in a function in the engine file that is used for the thrust variable here, so that I can be more detailed.

[Octal450]

@seanmcleod70