new caching of MagneticField in GlobalTracjetoryParameters - from M.Tadel
protection against null MF
Move verbose printout and add parameter safety
Merge pull request #55 from cerati/cerati/cacheMFinGTP

New caching of Magnetic Field in GlobalTrajectoryParamters class
Merge pull request #149 from mkirsano/mydevjuly2

Move verbose printout and add parameter safety