BUG: Calculation of `anz_erwachsene_tu` wrong

[MImmesberger]

Bug description

The current implementation of Soli relies on the number of individuals in the tax unit. The computation of anz_erwachsene_tu doesn't seem to work (see below). Seems not to be a huge issue as we are moving away from tu in the nearest future.

Reproducable example

import gettsim

print(f"GETTSIM version: {gettsim.__version__}")

params, functions = gettsim.set_up_policy_environment(2024)
test_df = gettsim.create_synthetic_data(n_adults=2)

results = gettsim.compute_taxes_and_transfers(
    test_df,
    params,
    functions,
    targets=["anz_erwachsene_tu"],
)

print(results)

Output:

GETTSIM version: 0.7.1.dev69+g3b8a62ea.d20240123
   anz_erwachsene_tu
0               True
1               True

[MImmesberger]

Closed via #694