Immutable searchspace

[AdrianSosic]

Fixes #371 by making SubspaceDiscrete stateless.

Current Approach

• The SubspaceDiscrete.metadata attribute gets deprecated and the responsibility of metadata handling is shifted to Campaign
• The new mechanism is not yet final (see out of scope below) but designed in a way that allows to implement upcoming changes in a non-breaking manner. In particular:
  • The metadata handling mechanism is redesigned in that the actual metadata representation is completely hidden from the user, i.e. campaign manages the data in form of private attributes. This is to avoid further lock-in into pandas as our search space backend and prepares for future search space improvements by abstracting away the specific implementation details, enabling us to easily offer other backends (polars, databases, etc) in the future.
  • The allow_* flags are not yet migrated to the Campaign class, but the AnnotatedSubspaceDiscrete allows to migrate them in a follow-up PR (Move allow_* flags to Campaign #423) without causing much friction
• A new user-facing method Campaign.toggle_discrete_candidates now allows convenient and dynamic control over the discrete candidate set, avoiding any fiddling with the backend dataframes and index manipulations. The positive effect can be seen in the much cleaner code parts of the simulation package.

Out of scope / (potentially) coming next

• Migration of allow_* flags in order to make Campaign the unique place where the concept of metadata exists, i.e. campaigns will be the only stateful objects. A PR taking care of this should follow soon because the get_candidates signature of SubspaceDiscrete currently makes not much sense, as it expects these flags in a context where metadata does not exist.
• Once the flags are migrated, the AnnotatedSubspaceDiscrete might become obsolete since the Campaign class can then theoretically filter down the space before passing it to the recommender. This however requires an efficient implementation that does not cause unnecessary dataframe copies.
• Actually turning the state space classes frozen. There a few other things that should be addressed at the same time (i.e. general cleanup of the classes).

[Scienfitz]

think this looks pretty nice
unlike in many other places I would rate the depreacation of metadata very important and it would be nice if it was functional (ie does what the old thing did plus warning) instead of error. Understand this might not be possible, just saying to perhaps catalyze more thought for this

[AVHopp]

Looks good, just some minot comments

[AVHopp]

Thanks for this important PR. Would appreciate if you could include the test dataframes as an example into the docstring of the toggle function but I can also live without that, hence already approve.

[AVHopp]

Happy with everything, would only like to hear your reasons for the singledispatchedmethod.