ENH: Raise warning if argument in create_synthetic_data is not among GETTSIM targets

[MImmesberger]

If a user overwrites the default target/input variable values in create_synthetic_data and has a typo in it, GETTSIM does not give a warning. This may lead to confusion, for example I had the case where:

hh_data = create_synthetic_data(
    n_adults=1,
    n_children=2,
    specs_heterogeneous={
        "kind_unterhalt_erhalt_m": [[0, i, i] for i in range (400, 800)]
    }
}

kind_unterhalt_erhalt_m is a typo, the correct target is kind_unterh_erhalt_m. In no step of the process (neither in i) creating the synthetic data nor in ii) calling gettsim) there is any notification/warning/error that may make the user aware of a potential typo.