Co-occurrence counts from Concordance [LUCENE-5318]

This patch calculates co-occurrence statistics on search terms within a window of x tokens. This can help in synonym discovery and anywhere else co-occurrence stats have been used.

The attached patch depends on #6381.

Again, many thanks to my colleague, Jason Robinson, for advice in developing this code and for his modifications to this code to make it more Solr-friendly.

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Migrated from [LUCENE-5318](https://issues.apache.org/jira/browse/LUCENE-5318) by Tim Allison (@tballison), 3 votes, updated Nov 19 2014
Attachments: [cooccur_v1.patch.gz](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5318/cooccur_v1.patch.gz)
Linked issues:
- #3823
- #6381
- [SOLR-5413](https://issues.apache.org/jira/browse/SOLR-5413)

Start with the attached cooccur_v1.patch.gz and linked issue #6381; no source files, tests, or entry points are named in the issue. Review the related #3823 and SOLR-5413 references, then determine how to calculate search-term co-occurrence within an x-token window and make the result Solr-friendly.

Written by the indexing model from the issue text.