Moving the flags into the flagger

Th flags DataFrame is now an instance variable of the flagger implementations. This is a mostly done WIP, only the plot tests and the new ContinuousBaseFlagger are not ported yet.

The flagger is now (mostly) immutable, i.e. everything returns a copy and leaves the calling flagger unchanged. This might sound expensive, but is (also to my surprise) actually not. The reason why the test suite takes longer now, is a non-ideal implementation of one test in funcs/test_functions.py. If we skip that, the time difference is minimal.

During the implementation of this, I came to the conclusion, that we should definitively move into this direction. While it makes some of the tests a bit more verbose, I think the reduction of verbosity throughout the rest of the code base is actually a worthwhile tradeoff.

I am a bit unsure if we should land this in %1.0 , any comments are highly appreciated @palmb !