update/improve module docstrings + add copyright notices

This MR improves the docstring of aas.adapter._generic and updates the docstring of aas.model.aas. The docstring of aas.adapter._generic is still quite short, but I think it contains all information that is necessary to understand what the module does / what its use is.
So this closes issues #61 (closed) and #62 (closed).

Additionally this MR adds missing copyright notices to:

• aas.adapter._generic
• aas.model.concept
• test._helper.setup_testdb