QOL & test improvements

[JonasScholl]

The first thing I recognised when using this package was that you don't get a proper error when the database URI is None. So I started just by adding type checks, but then I saw we could improve a bit more (seems like I'm not that good at code reviews at night in the train...)

So that's what I basically did:

• added type checks for input arguments
• restructured integration tests & added ones just for the database (besides the fastapi ones)
• the unit tests weren't really unit tests (they actually used the db) -> changed them to unit tests using mockito
• made the singletons argument-specific - for the case that you have multiple database configurations

[yannicschroeer]

Nice! A lot of useful improvements