DOC: ensure the documentation refers to CombinedNode, not ChainNode
DOC: minor grammar error
NF: allow for user-supplied initialization function
RF+BF+DOC: clarify code and ensure that wrong inputs crash the program even if __debug__ is not enabled
BF: use proper names for mean_measure and univariate_mean_measure
DOC: typo
BF: ensure that renaming of mean_measure and mean_univariate_measure is also applied to unit tests