ENH: Einkommensteuer before 2005

#### Is your feature request related to a problem?

This issue is supposed to serve as a parent issue for many smaller issues connected to extending GETTSIM's coverage of the income tax beyond 2005.

Currently, income taxes cannot be calculated before 2005 due to some bugs. We generally do not support income taxes before 2003 (we raise an error message if those are requested as targets).
