Input validation for all check_ functions

Currently, almost non of the `check_` -functions have any form of input validation. Consistent and complete input validation should be added, but performance should be monitored (`check_all()` should ideally not lead to input validation be performed 10 times...).

Locate all `check_` functions and the `check_all()` entry point first. Review how each function currently handles inputs and measure whether validation is repeated through `check_all()`. Done means consistent validation across the functions without unnecessary repeated validation.

Written by the indexing model from the issue text.