[ATHR] Set managed speed

Set the managed autothrust speed.

Mach:
Coherent.call("AP_MACH_VAR_SET", 2, mach);

Knots:
Coherent.call("AP_SPD_VAR_SET", 2, _speed);

Engage managed ATHR mode:
SimVar.SetSimVarValue("K:SPEED_SLOT_INDEX_SET", "number", 2);

Convert from knots to mach:
let mach = SimVar.GetGameVarValue("FROM KIAS TO MACH", "number", _speed);