Multiprocess new_ngram indexer column calculations?

[alexandermorgan]

Multiprocessing and the series_indexer were slated for removal in VIS 3 but it may be possible to multiprocess the new_ngram indexer's column calculations (each of which corresponds to an entire dataframe of the old ngram indexer). In theory this should be relatively easy but this is something that can wait until VIS 3.1 as it's not a major change. The new_ngram indexer is already relatively fast but it can never be too efficient because soon we'll start using it as an intermediary tool to help with complex things like dissonance detection.