static --> static const for GeometryStatics

This should fix warnings like this one

In file included from /build1/atnight/localbuilds/nightlies/master/athena/Tracking/TrkFitter/TrkGaussianSumFilter/src/GaussianSumFitter.cxx:21:
/build1/atnight/localbuilds/nightlies/master/athena/Tracking/TrkEvent/TrkEventUtils/TrkEventUtils/MeasurementBaseComparisonFunction.h: In member function 'double Trk::MeasurementBaseComparisonFunction::pathIntersectWithLine(const Trk::PerigeeSurface&) const':
/build1/atnight/localbuilds/nightlies/master/athena/Tracking/TrkEvent/TrkEventUtils/TrkEventUtils/MeasurementBaseComparisonFunction.h:191:48: warning: Static expression 'Trk::s_zAxis' of type 'Amg::Vector3D {aka Eigen::Matrix<double, 3, 1>}' passed to pointer or reference function argument of 'Eigen::MatrixBase<Eigen::Matrix<double, 3, 1> >::dot<Eigen::Matrix<double, 3, 1> >' within function 'double Trk::MeasurementBaseComparisonFunction::pathIntersectWithLine(const Trk::PerigeeSurface&) const'; may not be thread-safe.
       double     parallelity = m_direction->dot(Trk::s_zAxis);
                                ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

they started appearing after we added the static thread checker in a couple of packages. So I guess we catched a potential issue or so.

Mentioning @ssnyder and @oda that might have also seen these