TrkExSTEP_Propagator: Optimize some functions, even in dbg builds.
The function rungeKuttaStep makes heavy use of Eigen and runs extremely slowly in dbg builds. Although it was earlier forced to compile with optimiation, we didn't force eigen code to be inlined. Do that now. For the MuonCombinedConfig unit test, this results in a 30% speedup in the dbg build.