New VAT rates
On July 1st 2020, the VAT rates in Germany will be reduced until 2021: https://www.reuters.com/article/us-health-coronavirus-germany-economy-idUSKBN23J16Y
We need to update the tax definitions to get 3 children taxes: one for each period (before, during, after) for the taxes with 19% -> 16% and for 7% -> 5%.
Similar change has already happened in account_fr, see rev 58c224a204b3