ENH: Expand and improve parameters for Unterhaltsvorschuss

[cpz-research]

Is your feature request related to a problem?

GETTSIM produces an error whenever unterhaltsvors is calculated before 2017 since parameters in unterhaltsvors.yaml are not specified for the time before.

Before #544, the parameters were defined from 2016 on, but we somehow lost 1 year. See #479.

Describe the solution you would like to see

• Look for law references for the time before
• Expand time frame
• When working on it: altersgrenzen should be a list instead of a dict as the keys are meaningless as far as I understand.