Rename all variables named rval #1461
Description
We should rename all variables named rval because they are an abbreviation that is not immediately clear (return_value). Moreover, there's one occasion where the abbreviation stands for run_value (https://github.com/automl/auto-sklearn/blob/184b4dc279c498ed8eadeb620873b46e8230d83f/autosklearn/estimators.py), which could lead to errors in the future.