ACTS-101_enableRootThreadSafety

In order use root writers in a thread safe way one needs to use the global function ROOT::EnableThreadSafety() (see: https://root.cern.ch/doc/v606/namespaceROOT.html#a3332c2f629881ab608768fa6846f440e) at the beginning of each example. This solution was suggested when having problem to write out with two root writers simultaneously (see: https://root-forum.cern.ch/t/filling-separate-ttrees-in-parallel-threads/27175) Fixes ACTSFW-101.