Ref T231, Ref T236, Ref T238 improvements of interpolator

* keep last interpolated sitation, removed m_isFirstInterpolation
* allow to obtain the old/new situation
* transfer of elevations to newer situations

src/blackmisc/simulation/interpolatorspline.h

@@ -70,6 +70,12 @@ namespace BlackMisc
                 //! Interpolator for pitch, bank, heading, groundspeed
                 const CInterpolatorPbh &pbh() const { return m_pbh; }
 
+                //! Old situation
+                const Aviation::CAircraftSituation &getOldSituation() const { return pbh().getOldSituation(); }
+
+                //! New situation
+                const Aviation::CAircraftSituation &getNewSituation() const { return pbh().getNewSituation(); }
+
                 //! Set the time values
                 void setTimes(qint64 currentTimeMs, double timeFraction);