Use common script to get pacxx-samples for CI

Probably best implemented as a --samples option for getpacxx (!60 (merged)) to reuse the infrastructure of that script.

• Write the script (!64 (merged))
• use in project's CI (that has a working CI yet)
  • https://gitlab.dune-project.org/joe/pacxx-docker (!64 (merged), !65 (merged))
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx (N/A, really that repositories scripts should be tested)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-runtime (pacxx-runtime!36)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-samples (pacxx-samples!27)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-llvm2 (pacxx-llvm2!7)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-rv2 (pacxx-rv2!6)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-clang (pacxx-clang!4)