Evaluate the reach of recommender algorithms

The evaluation of a CF algorithm should include reach, the proportion of users for whom a recommendation could be made. An algorithm usually has a cutoff value on the confidence of the recommender, and if it is not high enough, no recommendation is made. The number of requested recommendations, or this parameter could be varied as part of the evaluation. The proposed patch adds this.

My build with this patch breaks testMapper(org.apache.mahout.classifier.df.mapreduce.partial.Step1MapperTest): org.apache.mahout.classifier.df.node.Leaf.<init>(I)V . The test seems unrelated to the patch, so I am assuming this is broken in the trunk head as well. Unfortunately I am under a deadline, and I do not have time to write tests for the patch.