Closes #538

Summary

adds support for sourcing systemtest tutorials from external git repositories and archives, instead of assuming all tutorials are in the local repo.

Changes

• sources.py: New module with TutorialSource, resolve_tutorial_root, fetch_git_repo, and fetch_archive for git and archive sources
• TestSuite.from_yaml: Parses optional source block per tutorial entry; fetches and loads external tutorials when not found locally
• Tutorial.source: Added source field (default: local) and optional base_dir in Tutorial.from_yaml and ReferenceResult
• Systemtest.__copy_tutorial_into_directory: Uses resolve_tutorial_root for all sources; PR/ref overrides only for local tutorials
• fetch_git_repo: Fallback when --branch fails—clone without branch, then try main → develop → master

Usage (tests.yaml)

• path: some-tutorial
  source:
  type: git
  url: https://github.com/precice/tutorials.git
  ref: develop
  subdir: tutorials # optional
  case_combination: [...]
  reference_result: ./some-tutorial/reference-results/...

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in changelog-entries/682.md.
• I will remember to squash-and-merge, providing a useful summary of the changes when merging.