Merge branch 'backref' into 'dev'

src/caoscrawler/crawl.py

src/caoscrawler/identifiable.py

src/caoscrawler/identifiable_adapters.py

src/doc/concepts.rst

synchronize.md

.../test_directories/single_file_test_data/identifiables.yml

unittests/test_file_identifiables.py

unittests/test_identifiable.py

unittests/test_identifiable_adapters.py

unittests/test_issues.py

unittests/test_tool.py