A common base (and also test_base) was defined for the whole application
So far, every module (contacts, variables, stationmeta, timeseries, and data) was defining its own base.
This was also done in pytest for test_base.
The base has now been harmonized to serve for the whole app. This has still to be done for the pytest environment.
Implicit (PostgreSQL) sequences were not automatically created by create_all, but sequences can be defined explicitly inside the db.models -- then there is no need to define them within the pytests (which looks somehow weird and makes the code hard to read).