BoundarySurface : Remove mutable, try to organise the access through members.

• Remove mutable

  mutable const Tvol*                 m_insideVolume;
     mutable const Tvol*                 m_outsideVolume;
     mutable SharedObject<VolumeArray>   m_insideVolumeArray;
     mutable SharedObject<VolumeArray>   m_outsideVolumeArray;

• These were accessed through "friendship" , in ways that were not 100% predictable when const objects are modified (which is not always nice for MT)

/** delcare the TrackingVolume Manipulator as friend */
    friend class TrackingVolumeManipulator;
    /** delcare the TrackingVolume as friend */
    friend class TrackingVolume;

• So organised the access via getters/setters. As an example :

/** getters/setters for inside/outside Volume*/
     Tvol const* insideVolume() const;
     void setInsideVolume(const Tvol*);
     void setInsideVolume ATLAS_NOT_CONST_THREAD_SAFE(const Tvol*) const ;

• The last one is for setting the insideVolume for a const BoundarySurface ... One should not do this except if knows that is not const. In which case the non-const overload is better and provided. But we have quite a bit of code out there during the Tracking Geometry building that in reality deals with initially non-const objects which are then passed around as const (so safe but not nice). Mark the case as ATLAS_NOT_CONST_THREAD_SAFE so clients get notified if they run the ATLAS thread safety checker.