RungeKuttaUtils : Move implementation functions to anonymous namespace to de-clutter header

https://twiki.cern.ch/twiki/bin/viewauth/AtlasComputing/CodingGuidelines#Static_and_global_objects

For entities which are not public but are also not really part of a class, prefer putting them in an anonymous namespace to putting them in a class. That way, they won't clutter up the header file.