Design a composable env script

[dennisklein]

• FairSoft should deliver an env script that populates all relevant $*_PATH search paths
  • Spack already provides spack load and optionally environment modules
• FairRoot and/or ExperimentRoot would then provide analog env scripts that just depend on the upstream ones
• One should consider to use an environment module system (e.g. lmod)
• Technically, there would also be a difference between loading a development and a runtime environment. However, as long as we use ROOT macros, we cannot distinguish the two, because macros need almost the full development environment at runtime.