Extension to MoreLikeThis to use tag information [LUCENE-1910]

I would like to contribute a class based on the MoreLikeThis class in
contrib/queries that generates a query based on the tags associated
with a document. The class assumes that documents are tagged with a
set of tags (which are stored in the index in a seperate Field). The
class determines the top document terms associated with a given tag
using the information gain metric.

While generating a MoreLikeThis query for a document the tags
associated with document are used to determine the terms in the query.
This class is useful for finding similar documents to a document that
does not have many relevant terms but was tagged.

---
Migrated from [LUCENE-1910](https://issues.apache.org/jira/browse/LUCENE-1910) by Thomas D'Silva, updated Aug 02 2010
Attachments: [LUCENE-1910.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-1910/LUCENE-1910.patch)

Start with MoreLikeThis in contrib/queries and review the attached LUCENE-1910.patch. Trace how tags are stored in a separate indexed field and how the proposed class selects terms using information gain. Done means a tag-aware MoreLikeThis query can use a document's tags to find similar documents with few relevant terms.

Written by the indexing model from the issue text.