refactor: Move PBH interpolator to inidivdual Interpolants

This allows each interpolant to specify on its own which PBH
interpolator should be used. Currently only the linear PBH interpolator
is available, but another PBH interpolator will be added for velocity.

This removes some protected data from the IInterpolator interface as well,
which conforms to C++ Core Guideline C.113 ("Avoid protected data").

src/blackmisc/simulation/interpolatorlinear.h

@@ -60,10 +60,13 @@ namespace BlackMisc
                 //! End situation
                 const Aviation::CAircraftSituation &getEndSituation() const { return m_endSituation; }
 
+                CInterpolatorPbh pbh() const { return m_pbh; }
+
             private:
                 Aviation::CAircraftSituation m_startSituation;
                 Aviation::CAircraftSituation m_endSituation;
                 double m_simulationTimeFraction = 0.0; //!< 0..1
+                CInterpolatorPbh m_pbh;
             };
 
             //! Get the interpolant for the given time point