[ATHR] Set selected speed

Used to set the autothrust selected speed.

Mach:
Coherent.call("AP_MACH_VAR_SET", 1, mach); SimVar.SetSimVarValue("K:AP_MANAGED_SPEED_IN_MACH_ON", "number", 1);

Knots:
Coherent.call("AP_SPD_VAR_SET", 1, speed); SimVar.SetSimVarValue("K:AP_MANAGED_SPEED_IN_MACH_OFF", "number", 1);

Set selected ATHR mode:
SimVar.SetSimVarValue("K:SPEED_SLOT_INDEX_SET", "number", 1);

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