Implement interpolator mode for FS9

Summary:
This implementation will always return true if we were setting the mode,
since we cannot easily access the return value from
CInterpolatorMulti::setMode in a thread safe manner.

Reviewers: #swift_pilot_client, kbasan, msutcliffe

Reviewed By: #swift_pilot_client, kbasan, msutcliffe

Subscribers: kbasan, jenkins

Differential Revision: https://dev.swift-project.org/D52

src/blackcore/simulator.h

@@ -164,6 +164,7 @@ namespace BlackCore
         virtual bool isShuttingDown() const = 0;
 
         //! Set interpolation mode, empty callsign applies to all know callsigns
+        //! \return Returns true if the mode changed, otherwise false. Note that some implementations always return true.
         virtual bool setInterpolatorMode(BlackMisc::Simulation::CInterpolatorMulti::Mode mode, const BlackMisc::Aviation::CCallsign &callsign) = 0;
 
         //! \addtogroup swiftdotcommands