BUG: Add last year's income as an input variable for Elterngeld

[MImmesberger]

Bug description

As discussed in #690, GETTSIM uses this year's income as an approximation for last year's income when calculating Elterngeld.

Instead, we should use last year's income as a basic input variable to correctly compute Elterngeld. At the same time, we should have the Elterngeld-relevant income as an output variable to facilitate computations in panel data.

[cpz-research]

There is a similar problem for grundrente which requires income from two (or three?) years ago.

See docstring of _grundr_zuschlag_eink_vor_freibetrag_m

[hmgaudecker]

And ALG I, in case we want to expand the scope here.