Include possibility for meta-calls in API

[hmgaudecker]

This has come up during the discussions on #778 and related.

The graph relies on a 1:1 mapping of function arguments and data. But we need a (n unknown) number candidate contents of bg_id and wthh_id in order to determine the correct setting in households where we have more than one fg and/or more than one bg.

#778 solves this via a complex workaround, which involves manually using fg_id as a candidate bg_id. It only works for a small number of candidates and which requires fg_id == hh_id.

Instead, we should provide for an option to

1. Call a part of the DAG / another function which returns all possible candidate bg_id-wthh_id constellations, which pass some "absolute" checks (e.g., we won't need to worry about households failing the wealth checks).
2. Create a sub-DAG, which will output the required targets (here, probably something like whether a particular candidate allocation of bg_ids and wthh_ids is valid in the first place along with eligibility for and payments from arbeitsl_geld_2, kinderzuschl, wohngeld).
3. Make checks like vorrang or günstiger based on the results from 2.

This will be complex, also for users. We may want to provide a shortcut that allows for the same setup as we used to have: bg_id == fg_id == wthh_id == hh_id, then the checks are trivial.