Add documentation and make class abstract

Added description to the method and made the class abstract

GBL/include/GblTrackSegmentController.h

@@ -45,9 +45,15 @@ namespace genfit {
     
     virtual ~GblTrackSegmentController() {};
     
-    virtual GblTrackSegmentController* clone() const {return new GblTrackSegmentController(*this);}
-    
-    virtual void controlTrackSegment(TVector3, TVector3, double, GblFitter *) {;}
+    /**
+    * @brief Function called for each segment of trajectory. User can decide on MS options.
+    *        This function must be implemented by the actual class deriving from this abstract class
+    * @param entry Position of segment starting point
+    * @param exit Position of segment ending point
+    * @param scatTheta Total MS variance accumulated in this segment
+    * @param fitter Pointer to the fitter - so you can set the MS options
+    */
+    virtual void controlTrackSegment(TVector3 entry, TVector3 exit, double scatTheta, GblFitter * fitter) = 0;
     
     virtual void Print(const Option_t* = "") const {;}