Register empty parentage up front

Currently an empty parentage vector is registered in the parentage registry every time a ROOT/EDM output file is closed.  This can cause thread safety problems in a multithreaded framework.  This pull request registers the empty parentage vector once up front during event processor initialization, and removes the registration from the output module.
This request also makes two C++ 11 improvements to RootOutputFile.cc
1) The use of range for loops when appropriate.
2) Use of nullptr rather than 0 when appropriate.