Fix two const correctness issues of ImpactPoint3dEstimator

Fix two const correctness issues of ImpactPoint3dEstimator by removing methods and mutable members.

http://goetz.web.cern.ch/goetz/mutable_2019-07-13.html#Other_Vx,

 nightly/master/2019-07-13T2149:Tracking/TrkVertexFitter/TrkVertexFitterUtils/src/ImpactPoint3dEstimator.h +111


   109	    double m_precision;
   110	    
   111	    mutable Amg::Vector3D* m_vertex;
   112	    
   113	    mutable double m_distance;

nightly/master/2019-07-13T2149:Tracking/TrkVertexFitter/TrkVertexFitterUtils/src/ImpactPoint3dEstimator.h +113


   111	    mutable Amg::Vector3D* m_vertex;
   112	    
   113	    mutable double m_distance;
   114	    
   115	

RunTier0Tests.py was fine. RunTier0Tests.log