C Make projects optional

[cmutel]

Projects as separate subdirectories is not a useful concept for all databases or organizations. Rather, they are an implementation detail of the current database backend structure (separate SQLite databases in subdirectories).

Specific tasks:

• C1 Separate project handling into a separate library
• C2 Adapt testing framework to not rely on projects
• C3 Provide separate test decorators for backends which do and do not use the projects concept

[cmutel]

Project handling is now in https://github.com/brightway-lca/brightway_projects.