word order for causal_words_pred

[bnicenboim]

@lisalevinson is concerned that the word order depends on row order:

ropensci/software-review#575 (comment)

Another concern is about the word/token order within groups/sentences. The word context/prefix is based purely on row order? It doesn't seem like the wordn value is used at all. This seems fragile if tokens possibly get sorted/rearranged, and it might not be an obvious problem if one is not carefully inspecting the order of tokens in the outputs. I know usually it will be fine but I always am nervous when anything in data analysis crucially depends on row order.

[bnicenboim]

fixed