Feature/Testing: Improve the validation / tests for finding NAs
UPdate on 14.3.2023: At the moment, the validation seemingly only takes 1 row to check? However, we saw that we can have severe NA issues in variables and ideally the check will take N rows to see whether it can fish out something fishy (höhö).
However, I do not know why Thomas used one row - maybe it has a reason and it will break when increasing to N rows.