Classes in DataFormats/L1DTTrackFinder only return const * from const functions

The thread-safety checker complained that the Container classes in DataFormats/L1DTTrackFinder
were returning non-const pointers to member data from const member functions. These were
changed to return const pointers. This required changing variable types in calling classes
however it turned out those classes never modified the objects anyway so no algorithmic
changes were neeed.