FieldConfiguration: Add LargestAcceptableStep, change field printout

This parameter is needed to not have single steps attempting to reach 1km (the
default, see
https://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/ch04s03.html
). This can happen for low energy curlers in the beam pipe vacuum.

Add this also to DDG4 setup and to documentation example for field
get default value in case it is not set