Fixes for lcg 104
- fixed stand-alone compilation of public headers (see LHCb!4211 (merged))
- some headers were fixed and some unused ones removed, in particular those containing syntax errors
- set the maximum C++ standard to 17 for GenEvent, because the version of Pythia8 we use does not work with C++20 (see also Gaussino/Gaussino!158 (merged))