We should delete NGramGenerator, as it is out of date:

- it does not use our Spotlight tokenizers
- it does not have the latest redirect definitions
- we should not duplicate

The disk-based method should use the same RestrictedNGramGenerator class (it can do both disk-based and memory-based), but with an empty String instead of the surface form lookup file to tell it it should not keep SFs in memory.

    DEFINE ngramGenerator pignlproc.helpers.RestrictedNGramGenerator('$MAX_NGRAM_LENGTH', '', '$LOCALE');