Add stem type filter for prefixes

[Kay-Michael Würzner]

Right now, productive prefixes can be added to manually prefixed words:

{"result": "zer<PREF>:<epsilon>aus<PREF>:<epsilon><epsilon>:g<epsilon>:eW:wert<NN>:<epsilon><CONV>:<epsilon>e:<epsilon>n:<epsilon><epsilon>:e<epsilon>:t<+V>:<epsilon><PPast>:<epsilon>", "query": "zerausgewertet"}

This could be prevented by applying a stem type filter to prefixing.