Remove decorator from python API (#8505)

Remove decorator from python API (#8505)

This PR removes the expand_list_arg decorator from the python API. It was used to allow calling a function f(x, *args) with a list as second argument and automatically expand the list into *args. While it merely allows for calling f(x, l) instead of f(x, *l), it adds considerable complexity to the code and documentation. Thus, following the Zen of python (have only one way to do it) we remove this decorator. This is also consistent with the pythonic API, were we made the same decision.