Use `keyword_expansion` tools in `keyword_filter` methodology

[jaklinger]

Currently there is no contextual expansion on the health lab keywords. Using the JLH algorithm (or other approach, though the JLH code is already in utils.keyword_expansion), it may be possible to include contextually relevant additional keywords.

You should also think about the relative importance of different keywords, since they currently all have equivalent weight. This could improve the ranking mechanism.