transform test functions to SaQC methods

A working (i.e. tests are passing) implementation for #181 (closed). The implementation (not the MR) is sort of minimal and more or less a reorganization of existing code/functions. However, I decided to removed core.register.FunctionWrapper in favor of a lengthy function containing all the parameter preparation and function calling logic. While this is not pretty, it was simpler for me to keep things together, while working on this.