Test and code refactoring

The refactoring is still far from perfect, but these are some first steps.

• removed lots of code duplication
• created helpers package in tests
• fixed some small bugs wrt. harvesters
• removed some useless dead code (and corresponding tests)
• removed lots of dumping and print commands polluting the test output
• improved some structures in the code

Notable changes:

• pydantic models for technical/provenance metadata (instead of plain dicts)
• instead of the tests/run_all_*.sh files that launch tests and must be run from tests, now tests can be run as is normal from the root directory, the old scripts are replaced with poe, i.e. poetry run poe test

Remaining problems (need discussion / support from @j.broeder):

• fix sitemap FZJ test (some URLs are apparently not mapped)
• discuss the technical metadata modelling