TRT_TrajectoryElement_xk, rm unneded dynamic_casts (ATLASRECTS-6547_item8) , use std math and some small tidy up
requested to merge ATLAS-EGamma/athena:ATLASRECTS-6547_item8_TRT_TrajectoryElement_xk_dyn_cast into master
TRT_TrajectoryElement_xk, rm unneded dynamic_casts (ATLASRECTS-6547_item8) , use std math and some small tidy up