Cleanup CMake configuration

[tmadlener]

The current CMake configuration can be cleaned up a bit I think. There are a few conventions that we usually use in Key4hep

• Follow the GnuInstallDirs which can be easily configured via CMake.
• Have CMakeLists.txt in subdirectories

I am also not entirely sure which parts of the current configuration from HepMC3 is still necessary.

This issue is mainly here for me to keep track and not forget about this.