Idf solution

When calculating tfidf for a term that is not in the corpus it returns 0, but calculating idf for the term is Infinity (division by zero).

This code caclucates idf according to the Lucene variation of idf. 
https://lucene.apache.org/core/2_9_4/api/all/org/apache/lucene/search/Similarity.html

                                numDocs
idf   =       	1 + log (	________  )
                                docFreq+1