Add reranking behavior metric functions

To support measuring the behavior of reranking components, it would be useful to add functions that will compute Rank Biased Overlap and Least Item Promoted given two lists (a base ranking and reranked outputs).

For the initial task (this issue), these should just be functions that take two ItemLists along with other parameters (i.e., the weighting model), and return the score.

When working on #766, we can work on an interface to incorporate these kinds of metrics into an analysis. Getting the code into LensKit as functions without integration into an analysis or evaluation loop will allow the code to at least be reused in prototype experiments while we work on the harder interface design problem.