def idf(word)
      idf = @idfs.get(word)
      if(idf.nil?)
        idf = @idfs.average
      end
      return idf
    end
def idf(word)
      idf = @idfs.get(word)
      if(idf.nil?)
        idf = @idfs.average
      end
      return idf
    end
IDFを取得する
 word:: 形態素
 return:: IDF