Reorganize, simplify and clean up fixtures

[gonzaponte]

We use many fixtures. Most of them are thrown into invisible_cities/conftest.py without thinking much. Let's reorganize them following the following logic:

• If they are used only in one file, move them to that file
• If they are used only in one folder, move them to the conftest.py file in that folder
• Otherwise, keep them in invisible_cities/conftest.py

Moreover, some of those fixtures are monstrous. Let's try to simplify them or replace them with test files whenever possible to keep things readable.