back to CMAKE_CXX_STANDARD 14
Looks like the project doesn't compile with ROOT with CMAKE_CXX_STANDARD=17 . https://gitlab.cern.ch/lhcb-parallelization/Allen/merge_requests/122#note_2577137
In this MR I moved it back to 14, also got rid of c++17 code.
@freiss Please let me know whether it works for you.
Edited by Daniil Pliushchenko