refactor: Use dynamic lookup with pure virtual methods

This makes it clearer what an IInterpolant and Interpolator must
implemented.
But it should be checked if using virtual here is okay performance-wise.

src/blackmisc/simulation/interpolatorlinear.cpp

@@ -100,7 +100,7 @@ namespace BlackMisc::Simulation
         }
     }
 
-    CInterpolatorLinear::CInterpolant CInterpolatorLinear::getInterpolant(SituationLog &log)
+    const IInterpolant &CInterpolatorLinear::getInterpolant(SituationLog &log)
     {
         // set default situations
         CAircraftSituation startSituation = m_interpolant.getStartSituation();